diff --git a/BUILD.gn b/BUILD.gn
index 4a7293262d802cf29c81c1b60ed97d0cca4ffda6..f92112e19a9f940b6581c1daf51d8fa741b7cd58 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -54,6 +54,7 @@ main_extern = [
   "$rust_build:tempfile",
   "$rust_build:rand",
   "$rust_build:tokio",
+  "$rust_build:tokio_executor",
   "$rust_build:url",
   "$rust_build:remove_dir_all",
   "$rust_build:dirs",
diff --git a/js/globals.ts b/js/globals.ts
index 3e40db06cdc3de8b81598f72cf7e833dec9ffa58..d88e1b28de8e267c36e364edb4574c183565b5b9 100644
--- a/js/globals.ts
+++ b/js/globals.ts
@@ -56,8 +56,6 @@ declare global {
 export const window = globalEval("this");
 window.window = window;
 
-window.libdeno = null;
-
 window.setTimeout = timers.setTimeout;
 window.setInterval = timers.setInterval;
 window.clearTimeout = timers.clearTimer;
diff --git a/src/handlers.rs b/src/handlers.rs
index 623c64110b28504e6e5d4922bc65fa8ba1e6f7f3..c914df36d0f9825b4dfedec1db08a4a10844ea0e 100644
--- a/src/handlers.rs
+++ b/src/handlers.rs
@@ -1,46 +1,44 @@
 // Copyright 2018 the Deno authors. All rights reserved. MIT license.
+
 use errors::DenoError;
 use errors::DenoResult;
-use flatbuffers::FlatBufferBuilder;
 use fs as deno_fs;
+use isolate::Buf;
+use isolate::IsolateState;
+use isolate::Op;
+use msg;
+
+use flatbuffers::FlatBufferBuilder;
 use futures;
 use futures::sync::oneshot;
 use hyper;
 use hyper::rt::{Future, Stream};
 use hyper::Client;
-use isolate::from_c;
-use libdeno;
-use libdeno::{deno_buf, isolate};
-use msg;
 use remove_dir_all::remove_dir_all;
 use std;
 use std::fs;
 #[cfg(any(unix))]
 use std::os::unix::fs::PermissionsExt;
 use std::path::Path;
+use std::sync::Arc;
 use std::time::UNIX_EPOCH;
 use std::time::{Duration, Instant};
 use tokio::timer::Delay;
 
-// Buf represents a byte array returned from a "Op".
-// The message might be empty (which will be translated into a null object on
-// the javascript side) or it is a heap allocated opaque sequence of bytes.
-// Usually a flatbuffer message.
-type Buf = Option<Box<[u8]>>;
-
-// JS promises in Deno map onto a specific Future
-// which yields either a DenoError or a byte array.
-type Op = Future<Item = Buf, Error = DenoError>;
-
 type OpResult = DenoResult<Buf>;
 
 // TODO Ideally we wouldn't have to box the Op being returned.
 // The box is just to make it easier to get a prototype refactor working.
-type Handler = fn(i: *const isolate, base: &msg::Base) -> Box<Op>;
+type Handler = fn(state: Arc<IsolateState>, base: &msg::Base) -> Box<Op>;
 
-pub extern "C" fn msg_from_js(i: *const isolate, buf: deno_buf) {
-  let bytes = unsafe { std::slice::from_raw_parts(buf.data_ptr, buf.data_len) };
+// Hopefully Rust optimizes this away.
+fn empty_buf() -> Buf {
+  Box::new([])
+}
+
+pub fn msg_from_js(state: Arc<IsolateState>, bytes: &[u8]) -> (bool, Box<Op>) {
   let base = msg::get_root_as_base(bytes);
+  let is_sync = base.sync();
   let msg_type = base.msg_type();
   let cmd_id = base.cmd_id();
   let handler: Handler = match msg_type {
@@ -68,73 +66,51 @@ pub extern "C" fn msg_from_js(i: *const isolate, buf: deno_buf) {
     )),
   };
 
-  let future = handler(i, &base);
-  let future = future.or_else(move |err| {
-    // No matter whether we got an Err or Ok, we want a serialized message to
-    // send back. So transform the DenoError into a deno_buf.
-    let builder = &mut FlatBufferBuilder::new();
-    let errmsg_offset = builder.create_string(&format!("{}", err));
-    Ok(serialize_response(
-      cmd_id,
-      builder,
-      msg::BaseArgs {
-        error: Some(errmsg_offset),
-        error_kind: err.kind(),
-        ..Default::default()
-      },
-    ))
-  });
-
-  let isolate = from_c(i);
-  if base.sync() {
-    // Execute future synchronously.
-    // println!("sync handler {}", msg::enum_name_any(msg_type));
-    let maybe_box_u8 = future.wait().unwrap();
-    match maybe_box_u8 {
-      None => {}
-      Some(box_u8) => {
-        let buf = deno_buf_from(box_u8);
-        // Set the synchronous response, the value returned from isolate.send().
-        unsafe { libdeno::deno_set_response(i, buf) }
-      }
-    }
-  } else {
-    // Execute future asynchornously.
-    let future = future.and_then(move |maybe_box_u8| {
-      let buf = match maybe_box_u8 {
-        Some(box_u8) => deno_buf_from(box_u8),
-        None => {
-          // async RPCs that return None still need to
-          // send a message back to signal completion.
-          let builder = &mut FlatBufferBuilder::new();
-          deno_buf_from(
-            serialize_response(
-              cmd_id,
-              builder,
-              msg::BaseArgs {
-                ..Default::default()
-              },
-            ).unwrap(),
-          )
-        }
+  let op: Box<Op> = handler(state.clone(), &base);
+  let boxed_op = Box::new(
+    op.or_else(move |err: DenoError| -> DenoResult<Buf> {
+      debug!("op err {}", err);
+      // No matter whether we got an Err or Ok, we want a serialized message to
+      // send back. So transform the DenoError into a deno_buf.
+      let builder = &mut FlatBufferBuilder::new();
+      let errmsg_offset = builder.create_string(&format!("{}", err));
+      Ok(serialize_response(
+        cmd_id,
+        builder,
+        msg::BaseArgs {
+          error: Some(errmsg_offset),
+          error_kind: err.kind(),
+          ..Default::default()
+        },
+      ))
+    }).and_then(move |buf: Buf| -> DenoResult<Buf> {
+      // Handle empty responses. For sync responses we just want
+      // to send null. For async we want to send a small message
+      // with the cmd_id.
+      let buf = if is_sync || buf.len() > 0 {
+        buf
+      } else {
+        // async RPCs that return empty still need to
+        // send a message back to signal completion.
+        let builder = &mut FlatBufferBuilder::new();
+        serialize_response(
+          cmd_id,
+          builder,
+          msg::BaseArgs {
+            ..Default::default()
+          },
+        )
       };
-      // TODO(ry) make this thread safe.
-      unsafe { libdeno::deno_send(i, buf) };
-      Ok(())
-    });
-    isolate.rt.spawn(future);
-  }
-}
+      Ok(buf)
+    }),
+  );
 
-fn deno_buf_from(x: Box<[u8]>) -> deno_buf {
-  let len = x.len();
-  let ptr = Box::into_raw(x);
-  deno_buf {
-    alloc_ptr: 0 as *mut u8,
-    alloc_len: 0,
-    data_ptr: ptr as *mut u8,
-    data_len: len,
-  }
+  debug!(
+    "msg_from_js {} sync {}",
+    msg::enum_name_any(msg_type),
+    base.sync()
+  );
+  return (base.sync(), boxed_op);
 }
 
 fn permission_denied() -> DenoError {
@@ -151,16 +127,15 @@ fn not_implemented() -> DenoError {
   ))
 }
 
-fn handle_exit(_i: *const isolate, base: &msg::Base) -> Box<Op> {
+fn handle_exit(_config: Arc<IsolateState>, base: &msg::Base) -> Box<Op> {
   let msg = base.msg_as_exit().unwrap();
   std::process::exit(msg.code())
 }
 
-fn handle_start(i: *const isolate, base: &msg::Base) -> Box<Op> {
-  let isolate = from_c(i);
+fn handle_start(state: Arc<IsolateState>, base: &msg::Base) -> Box<Op> {
   let mut builder = FlatBufferBuilder::new();
 
-  let argv = isolate.argv.iter().map(|s| s.as_str()).collect::<Vec<_>>();
+  let argv = state.argv.iter().map(|s| s.as_str()).collect::<Vec<_>>();
   let argv_off = builder.create_vector_of_strings(argv.as_slice());
 
   let cwd_path = std::env::current_dir().unwrap();
@@ -172,8 +147,8 @@ fn handle_start(i: *const isolate, base: &msg::Base) -> Box<Op> {
     &msg::StartResArgs {
       cwd: Some(cwd_off),
       argv: Some(argv_off),
-      debug_flag: isolate.flags.log_debug,
-      recompile_flag: isolate.flags.recompile,
+      debug_flag: state.flags.log_debug,
+      recompile_flag: state.flags.recompile,
       ..Default::default()
     },
   );
@@ -200,7 +175,7 @@ fn serialize_response(
   let data = builder.finished_data();
   // println!("serialize_response {:x?}", data);
   let vec = data.to_vec();
-  Some(vec.into_boxed_slice())
+  vec.into_boxed_slice()
 }
 
 fn ok_future(buf: Buf) -> Box<Op> {
@@ -213,22 +188,17 @@ fn odd_future(err: DenoError) -> Box<Op> {
 }
 
 // https://github.com/denoland/isolate/blob/golang/os.go#L100-L154
-fn handle_code_fetch(i: *const isolate, base: &msg::Base) -> Box<Op> {
+fn handle_code_fetch(state: Arc<IsolateState>, base: &msg::Base) -> Box<Op> {
   let msg = base.msg_as_code_fetch().unwrap();
   let cmd_id = base.cmd_id();
   let module_specifier = msg.module_specifier().unwrap();
   let containing_file = msg.containing_file().unwrap();
-  let isolate = from_c(i);
 
-  assert_eq!(
-    isolate.dir.root.join("gen"),
-    isolate.dir.gen,
-    "Sanity check"
-  );
+  assert_eq!(state.dir.root.join("gen"), state.dir.gen, "Sanity check");
 
   Box::new(futures::future::result(|| -> OpResult {
     let builder = &mut FlatBufferBuilder::new();
-    let out = isolate.dir.code_fetch(module_specifier, containing_file)?;
+    let out = state.dir.code_fetch(module_specifier, containing_file)?;
     let mut msg_args = msg::CodeFetchResArgs {
       module_name: Some(builder.create_string(&out.module_name)),
       filename: Some(builder.create_string(&out.filename)),
@@ -255,36 +225,34 @@ fn handle_code_fetch(i: *const isolate, base: &msg::Base) -> Box<Op> {
 }
 
 // https://github.com/denoland/isolate/blob/golang/os.go#L156-L169
-fn handle_code_cache(i: *const isolate, base: &msg::Base) -> Box<Op> {
+fn handle_code_cache(state: Arc<IsolateState>, base: &msg::Base) -> Box<Op> {
   let msg = base.msg_as_code_cache().unwrap();
   let filename = msg.filename().unwrap();
   let source_code = msg.source_code().unwrap();
   let output_code = msg.output_code().unwrap();
   Box::new(futures::future::result(|| -> OpResult {
-    let isolate = from_c(i);
-    isolate.dir.code_cache(filename, source_code, output_code)?;
-    Ok(None)
+    state.dir.code_cache(filename, source_code, output_code)?;
+    Ok(empty_buf())
   }()))
 }
 
-fn handle_set_env(i: *const isolate, base: &msg::Base) -> Box<Op> {
+fn handle_set_env(state: Arc<IsolateState>, base: &msg::Base) -> Box<Op> {
   let msg = base.msg_as_set_env().unwrap();
   let key = msg.key().unwrap();
   let value = msg.value().unwrap();
 
-  let isolate = from_c(i);
-  if !isolate.flags.allow_env {
+  if !state.flags.allow_env {
     return odd_future(permission_denied());
   }
 
   std::env::set_var(key, value);
-  ok_future(None)
+  ok_future(empty_buf())
 }
 
-fn handle_env(i: *const isolate, base: &msg::Base) -> Box<Op> {
-  let isolate = from_c(i);
+fn handle_env(state: Arc<IsolateState>, base: &msg::Base) -> Box<Op> {
   let cmd_id = base.cmd_id();
-  if !isolate.flags.allow_env {
+
+  if !state.flags.allow_env {
     return odd_future(permission_denied());
   }
 
@@ -322,22 +290,23 @@ fn handle_env(i: *const isolate, base: &msg::Base) -> Box<Op> {
   ))
 }
 
-fn handle_fetch_req(i: *const isolate, base: &msg::Base) -> Box<Op> {
+fn handle_fetch_req(state: Arc<IsolateState>, base: &msg::Base) -> Box<Op> {
   let msg = base.msg_as_fetch_req().unwrap();
   let cmd_id = base.cmd_id();
   let id = msg.id();
   let url = msg.url().unwrap();
-  let isolate = from_c(i);
 
-  if !isolate.flags.allow_net {
+  if !state.flags.allow_net {
     return odd_future(permission_denied());
   }
 
   let url = url.parse::<hyper::Uri>().unwrap();
   let client = Client::new();
 
+  debug!("Before fetch {}", url);
   let future = client.get(url).and_then(move |res| {
     let status = res.status().as_u16() as i32;
+    debug!("fetch {}", status);
 
     let headers = {
       let map = res.headers();
@@ -361,6 +330,7 @@ fn handle_fetch_req(i: *const isolate, base: &msg::Base) -> Box<Op> {
 
   let future = future.map_err(|err| -> DenoError { err.into() }).and_then(
     move |(status, body, headers)| {
+      debug!("fetch body ");
       let builder = &mut FlatBufferBuilder::new();
       // Send the first message without a body. This is just to indicate
       // what status code.
@@ -422,7 +392,7 @@ where
   (delay_task, cancel_tx)
 }
 
-fn handle_make_temp_dir(i: *const isolate, base: &msg::Base) -> Box<Op> {
+fn handle_make_temp_dir(state: Arc<IsolateState>, base: &msg::Base) -> Box<Op> {
   let base = Box::new(*base);
   let msg = base.msg_as_make_temp_dir().unwrap();
   let cmd_id = base.cmd_id();
@@ -430,8 +400,7 @@ fn handle_make_temp_dir(i: *const isolate, base: &msg::Base) -> Box<Op> {
   let prefix = msg.prefix();
   let suffix = msg.suffix();
 
-  let isolate = from_c(i);
-  if !isolate.flags.allow_write {
+  if !state.flags.allow_write {
     return odd_future(permission_denied());
   }
   // TODO Use blocking() here.
@@ -461,28 +430,28 @@ fn handle_make_temp_dir(i: *const isolate, base: &msg::Base) -> Box<Op> {
   }()))
 }
 
-fn handle_mkdir(i: *const isolate, base: &msg::Base) -> Box<Op> {
+fn handle_mkdir(state: Arc<IsolateState>, base: &msg::Base) -> Box<Op> {
   let msg = base.msg_as_mkdir().unwrap();
   let mode = msg.mode();
   let path = msg.path().unwrap();
-  let isolate = from_c(i);
-  if !isolate.flags.allow_write {
+
+  if !state.flags.allow_write {
     return odd_future(permission_denied());
   }
   // TODO Use tokio_threadpool.
   Box::new(futures::future::result(|| -> OpResult {
     debug!("handle_mkdir {}", path);
     deno_fs::mkdir(Path::new(path), mode)?;
-    Ok(None)
+    Ok(empty_buf())
   }()))
 }
 
-fn handle_remove(i: *const isolate, base: &msg::Base) -> Box<Op> {
+fn handle_remove(state: Arc<IsolateState>, base: &msg::Base) -> Box<Op> {
   let msg = base.msg_as_remove().unwrap();
   let path = msg.path().unwrap();
   let recursive = msg.recursive();
-  let isolate = from_c(i);
-  if !isolate.flags.allow_write {
+
+  if !state.flags.allow_write {
     return odd_future(permission_denied());
   }
   // TODO Use tokio_threadpool.
@@ -499,12 +468,12 @@ fn handle_remove(i: *const isolate, base: &msg::Base) -> Box<Op> {
         fs::remove_dir(&path_)?;
       }
     }
-    Ok(None)
+    Ok(empty_buf())
   }()))
 }
 
 // Prototype https://github.com/denoland/isolate/blob/golang/os.go#L171-L184
-fn handle_read_file(_i: *const isolate, base: &msg::Base) -> Box<Op> {
+fn handle_read_file(_config: Arc<IsolateState>, base: &msg::Base) -> Box<Op> {
   let msg = base.msg_as_read_file().unwrap();
   let cmd_id = base.cmd_id();
   let filename = String::from(msg.filename().unwrap());
@@ -554,7 +523,7 @@ fn get_mode(_perm: fs::Permissions) -> u32 {
   0
 }
 
-fn handle_stat(_i: *const isolate, base: &msg::Base) -> Box<Op> {
+fn handle_stat(_config: Arc<IsolateState>, base: &msg::Base) -> Box<Op> {
   let msg = base.msg_as_stat().unwrap();
   let cmd_id = base.cmd_id();
   let filename = String::from(msg.filename().unwrap());
@@ -597,48 +566,49 @@ fn handle_stat(_i: *const isolate, base: &msg::Base) -> Box<Op> {
   }()))
 }
 
-fn handle_write_file(i: *const isolate, base: &msg::Base) -> Box<Op> {
+fn handle_write_file(state: Arc<IsolateState>, base: &msg::Base) -> Box<Op> {
   let msg = base.msg_as_write_file().unwrap();
   let filename = String::from(msg.filename().unwrap());
   let data = msg.data().unwrap();
   let perm = msg.perm();
 
-  let isolate = from_c(i);
-  if !isolate.flags.allow_write {
+  if !state.flags.allow_write {
     return odd_future(permission_denied());
   }
+
   Box::new(futures::future::result(|| -> OpResult {
     debug!("handle_write_file {}", filename);
     deno_fs::write_file(Path::new(&filename), data, perm)?;
-    Ok(None)
+    Ok(empty_buf())
   }()))
 }
 
-fn remove_timer(i: *const isolate, timer_id: u32) {
-  let isolate = from_c(i);
-  isolate.timers.remove(&timer_id);
+fn remove_timer(state: Arc<IsolateState>, timer_id: u32) {
+  let mut timers = state.timers.lock().unwrap();
+  timers.remove(&timer_id);
 }
 
 // Prototype: https://github.com/ry/isolate/blob/golang/timers.go#L25-L39
-fn handle_timer_start(i: *const isolate, base: &msg::Base) -> Box<Op> {
+fn handle_timer_start(state: Arc<IsolateState>, base: &msg::Base) -> Box<Op> {
   debug!("handle_timer_start");
   let msg = base.msg_as_timer_start().unwrap();
   let cmd_id = base.cmd_id();
   let timer_id = msg.id();
   let delay = msg.delay();
-  let isolate = from_c(i);
 
+  let config2 = state.clone();
   let future = {
     let (delay_task, cancel_delay) = set_timeout(
       move || {
-        remove_timer(i, timer_id);
+        remove_timer(config2, timer_id);
       },
       delay,
     );
-    isolate.timers.insert(timer_id, cancel_delay);
+    let mut timers = state.timers.lock().unwrap();
+    timers.insert(timer_id, cancel_delay);
     delay_task
   };
-  Box::new(future.then(move |result| {
+  let r = Box::new(future.then(move |result| {
     let builder = &mut FlatBufferBuilder::new();
     let msg = msg::TimerReady::create(
       builder,
@@ -657,20 +627,20 @@ fn handle_timer_start(i: *const isolate, base: &msg::Base) -> Box<Op> {
         ..Default::default()
       },
     ))
-  }))
+  }));
+  r
 }
 
 // Prototype: https://github.com/ry/isolate/blob/golang/timers.go#L40-L43
-fn handle_timer_clear(i: *const isolate, base: &msg::Base) -> Box<Op> {
+fn handle_timer_clear(state: Arc<IsolateState>, base: &msg::Base) -> Box<Op> {
   let msg = base.msg_as_timer_clear().unwrap();
   debug!("handle_timer_clear");
-  remove_timer(i, msg.id());
-  ok_future(None)
+  remove_timer(state, msg.id());
+  ok_future(empty_buf())
 }
 
-fn handle_rename(i: *const isolate, base: &msg::Base) -> Box<Op> {
-  let isolate = from_c(i);
-  if !isolate.flags.allow_write {
+fn handle_rename(state: Arc<IsolateState>, base: &msg::Base) -> Box<Op> {
+  if !state.flags.allow_write {
     return odd_future(permission_denied());
   }
   let msg = base.msg_as_rename().unwrap();
@@ -679,13 +649,12 @@ fn handle_rename(i: *const isolate, base: &msg::Base) -> Box<Op> {
   Box::new(futures::future::result(|| -> OpResult {
     debug!("handle_rename {} {}", oldpath, newpath);
     fs::rename(Path::new(&oldpath), Path::new(&newpath))?;
-    Ok(None)
+    Ok(empty_buf())
   }()))
 }
 
-fn handle_symlink(i: *const isolate, base: &msg::Base) -> Box<Op> {
-  let deno = from_c(i);
-  if !deno.flags.allow_write {
+fn handle_symlink(state: Arc<IsolateState>, base: &msg::Base) -> Box<Op> {
+  if !state.flags.allow_write {
     return odd_future(permission_denied());
   }
   // TODO Use type for Windows.
@@ -699,12 +668,12 @@ fn handle_symlink(i: *const isolate, base: &msg::Base) -> Box<Op> {
       debug!("handle_symlink {} {}", oldname, newname);
       #[cfg(any(unix))]
       std::os::unix::fs::symlink(Path::new(&oldname), Path::new(&newname))?;
-      Ok(None)
+      Ok(empty_buf())
     }()))
   }
 }
 
-fn handle_read_link(_i: *const isolate, base: &msg::Base) -> Box<Op> {
+fn handle_read_link(_state: Arc<IsolateState>, base: &msg::Base) -> Box<Op> {
   let msg = base.msg_as_readlink().unwrap();
   let cmd_id = base.cmd_id();
   let name = String::from(msg.name().unwrap());
diff --git a/src/http.rs b/src/http.rs
index 5907b35ed66cf00be6cb33fdce7acb6c1a5f7103..3b5ede10e3fef10161c9f6f45bc7acc2e4ced405 100644
--- a/src/http.rs
+++ b/src/http.rs
@@ -1,10 +1,12 @@
 // Copyright 2018 the Deno authors. All rights reserved. MIT license.
 
 use errors::DenoResult;
+use tokio_util;
 
 use futures::Future;
 use futures::Stream;
 use hyper;
+use hyper::client::Client;
 use hyper::client::HttpConnector;
 use hyper::Uri;
 use hyper_rustls;
@@ -29,21 +31,24 @@ pub fn get_client() -> Client<Connector, hyper::Body> {
 pub fn fetch_sync_string(module_name: &str) -> DenoResult<String> {
   let url = module_name.parse::<Uri>().unwrap();
   let client = get_client();
-
-  // TODO Use Deno's RT
-  let mut rt = Runtime::new().unwrap();
-  let body = rt.block_on(
-    client
-      .get(url)
-      .and_then(|response| response.into_body().concat2()),
-  )?;
+  let future = client
+    .get(url)
+    .and_then(|response| response.into_body().concat2());
+  let body = tokio_util::block_on(future)?;
   Ok(String::from_utf8(body.to_vec()).unwrap())
 }
 
 #[test]
 fn test_fetch_sync_string() {
   // Relies on external http server. See tools/http_server.py
-  let p = fetch_sync_string("http://localhost:4545/package.json").unwrap();
-  println!("package.json len {}", p.len());
-  assert!(p.len() > 1);
+  use futures;
+
+  tokio_util::init(|| {
+    tokio_util::block_on(futures::future::lazy(|| -> DenoResult<()> {
+      let p = fetch_sync_string("http://127.0.0.1:4545/package.json")?;
+      println!("package.json len {}", p.len());
+      assert!(p.len() > 1);
+      Ok(())
+    })).unwrap();
+  });
 }
diff --git a/src/isolate.rs b/src/isolate.rs
index 5daf45701b72310ed5ab8dc88b4775bde769a69e..64bec5ddf11c9f811866f20934e0caf5796675ec 100644
--- a/src/isolate.rs
+++ b/src/isolate.rs
@@ -1,54 +1,113 @@
 // Copyright 2018 the Deno authors. All rights reserved. MIT license.
+
+// Do not use FlatBuffers in this module.
+// TODO Currently this module uses Tokio, but it would be nice if they were
+// decoupled.
+
 use deno_dir;
+use errors::DenoError;
 use flags;
+use libdeno;
+
 use futures;
-use handlers;
+use futures::Future;
 use libc::c_void;
-use libdeno;
 use std;
 use std::collections::HashMap;
 use std::ffi::CStr;
 use std::ffi::CString;
+use std::sync::mpsc;
+use std::sync::Arc;
+use std::sync::Mutex;
 use tokio;
+use tokio_util;
 
 type DenoException<'a> = &'a str;
 
+// Buf represents a byte array returned from a "Op".
+// The message might be empty (which will be translated into a null object on
+// the javascript side) or it is a heap allocated opaque sequence of bytes.
+// Usually a flatbuffer message.
+pub type Buf = Box<[u8]>;
+
+// JS promises in Deno map onto a specific Future
+// which yields either a DenoError or a byte array.
+pub type Op = Future<Item = Buf, Error = DenoError> + Send;
+
+// Returns (is_sync, op)
+pub type Dispatch = fn(state: Arc<IsolateState>, buf: &[u8]) -> (bool, Box<Op>);
+
 pub struct Isolate {
-  pub ptr: *const libdeno::isolate,
+  ptr: *const libdeno::isolate,
+  dispatch: Dispatch,
+  rx: mpsc::Receiver<Buf>,
+  ntasks: i32,
+  pub state: Arc<IsolateState>,
+}
+
+// Isolate cannot be passed between threads but IsolateState can. So any state that
+// needs to be accessed outside the main V8 thread should be inside IsolateState.
+pub struct IsolateState {
   pub dir: deno_dir::DenoDir,
-  pub rt: tokio::runtime::current_thread::Runtime,
-  pub timers: HashMap<u32, futures::sync::oneshot::Sender<()>>,
+  pub timers: Mutex<HashMap<u32, futures::sync::oneshot::Sender<()>>>,
   pub argv: Vec<String>,
   pub flags: flags::DenoFlags,
+  tx: Mutex<Option<mpsc::Sender<Buf>>>,
+}
+
+impl IsolateState {
+  // Thread safe.
+  fn send_to_js(&self, buf: Buf) {
+    let mut g = self.tx.lock().unwrap();
+    let maybe_tx = g.as_mut();
+    assert!(maybe_tx.is_some(), "Expected tx to not be deleted.");
+    let tx = maybe_tx.unwrap();
+    tx.send(buf).expect("tx.send error");
+  }
 }
 
 static DENO_INIT: std::sync::Once = std::sync::ONCE_INIT;
 
 impl Isolate {
-  pub fn new(argv: Vec<String>) -> Box<Isolate> {
+  pub fn new(argv: Vec<String>, dispatch: Dispatch) -> Box<Isolate> {
     DENO_INIT.call_once(|| {
       unsafe { libdeno::deno_init() };
     });
 
     let (flags, argv_rest) = flags::set_flags(argv);
 
-    let mut deno_box = Box::new(Isolate {
+    // This channel handles sending async messages back to the runtime.
+    let (tx, rx) = mpsc::channel::<Buf>();
+
+    let mut isolate = Box::new(Isolate {
       ptr: 0 as *const libdeno::isolate,
-      dir: deno_dir::DenoDir::new(flags.reload, None).unwrap(),
-      rt: tokio::runtime::current_thread::Runtime::new().unwrap(),
-      timers: HashMap::new(),
-      argv: argv_rest,
-      flags,
+      dispatch,
+      rx,
+      ntasks: 0,
+      state: Arc::new(IsolateState {
+        dir: deno_dir::DenoDir::new(flags.reload, None).unwrap(),
+        timers: Mutex::new(HashMap::new()),
+        argv: argv_rest,
+        flags,
+        tx: Mutex::new(Some(tx)),
+      }),
     });
 
-    (*deno_box).ptr = unsafe {
+    (*isolate).ptr = unsafe {
       libdeno::deno_new(
-        deno_box.as_ref() as *const _ as *const c_void,
-        handlers::msg_from_js,
+        isolate.as_ref() as *const _ as *const c_void,
+        pre_dispatch,
       )
     };
 
-    deno_box
+    isolate
+  }
+
+  pub fn from_c<'a>(d: *const libdeno::isolate) -> &'a mut Isolate {
+    let ptr = unsafe { libdeno::deno_get_data(d) };
+    let ptr = ptr as *mut Isolate;
+    let isolate_box = unsafe { Box::from_raw(ptr) };
+    Box::leak(isolate_box)
   }
 
   pub fn execute(
@@ -68,6 +127,42 @@ impl Isolate {
     }
     Ok(())
   }
+
+  pub fn set_response(&self, buf: Buf) {
+    unsafe { libdeno::deno_set_response(self.ptr, buf.into()) }
+  }
+
+  pub fn send(&self, buf: Buf) {
+    unsafe { libdeno::deno_send(self.ptr, buf.into()) };
+  }
+
+  // TODO Use Park abstraction? Note at time of writing Tokio default runtime
+  // does not have new_with_park().
+  pub fn event_loop(&mut self) {
+    // Main thread event loop.
+    while !self.is_idle() {
+      let buf = self.rx.recv().unwrap();
+      // Receiving a message on rx exactly corresponds to an async task
+      // completing.
+      self.ntasks_decrement();
+      // Call into JS with the buf.
+      self.send(buf);
+    }
+  }
+
+  fn ntasks_increment(&mut self) {
+    assert!(self.ntasks >= 0);
+    self.ntasks = self.ntasks + 1;
+  }
+
+  fn ntasks_decrement(&mut self) {
+    self.ntasks = self.ntasks - 1;
+    assert!(self.ntasks >= 0);
+  }
+
+  fn is_idle(&self) -> bool {
+    self.ntasks == 0
+  }
 }
 
 impl Drop for Isolate {
@@ -76,22 +171,107 @@ impl Drop for Isolate {
   }
 }
 
-pub fn from_c<'a>(i: *const libdeno::isolate) -> &'a mut Isolate {
-  let ptr = unsafe { libdeno::deno_get_data(i) };
-  let ptr = ptr as *mut Isolate;
-  let isolate_box = unsafe { Box::from_raw(ptr) };
-  Box::leak(isolate_box)
+/// Converts Rust Buf to libdeno deno_buf.
+impl From<Buf> for libdeno::deno_buf {
+  fn from(x: Buf) -> libdeno::deno_buf {
+    let len = x.len();
+    let ptr = Box::into_raw(x);
+    libdeno::deno_buf {
+      alloc_ptr: 0 as *mut u8,
+      alloc_len: 0,
+      data_ptr: ptr as *mut u8,
+      data_len: len,
+    }
+  }
 }
 
-#[test]
-fn test_c_to_rust() {
-  let argv = vec![String::from("./deno"), String::from("hello.js")];
-  let isolate = Isolate::new(argv);
-  let isolate2 = from_c(isolate.ptr);
-  assert_eq!(isolate.ptr, isolate2.ptr);
-  assert_eq!(
-    isolate.dir.root.join("gen"),
-    isolate.dir.gen,
-    "Sanity check"
-  );
+// Dereferences the C pointer into the Rust Isolate object.
+extern "C" fn pre_dispatch(d: *const libdeno::isolate, buf: libdeno::deno_buf) {
+  let bytes = unsafe { std::slice::from_raw_parts(buf.data_ptr, buf.data_len) };
+  let isolate = Isolate::from_c(d);
+  let dispatch = isolate.dispatch;
+  let (is_sync, op) = dispatch(isolate.state.clone(), bytes);
+
+  if is_sync {
+    // Execute op synchronously.
+    let buf = tokio_util::block_on(op).unwrap();
+    if buf.len() != 0 {
+      // Set the synchronous response, the value returned from isolate.send().
+      isolate.set_response(buf);
+    }
+  } else {
+    // Execute op asynchronously.
+    let state = isolate.state.clone();
+
+    // TODO Ideally Tokio would could tell us how many tasks are executing, but
+    // it cannot currently. Therefore we track top-level promises/tasks
+    // manually.
+    isolate.ntasks_increment();
+
+    let task = op
+      .and_then(move |buf| {
+        state.send_to_js(buf);
+        Ok(())
+      }).map_err(|_| ());
+    tokio::spawn(task);
+  }
+}
+
+#[cfg(test)]
+mod tests {
+  use super::*;
+
+  #[test]
+  fn test_c_to_rust() {
+    let argv = vec![String::from("./deno"), String::from("hello.js")];
+    let isolate = Isolate::new(argv, unreachable_dispatch);
+    let isolate2 = Isolate::from_c(isolate.ptr);
+    assert_eq!(isolate.ptr, isolate2.ptr);
+    assert_eq!(
+      isolate.state.dir.root.join("gen"),
+      isolate.state.dir.gen,
+      "Sanity check"
+    );
+  }
+
+  fn unreachable_dispatch(
+    _state: Arc<IsolateState>,
+    _buf: &[u8],
+  ) -> (bool, Box<Op>) {
+    unreachable!();
+  }
+
+  #[test]
+  fn test_dispatch_sync() {
+    let argv = vec![String::from("./deno"), String::from("hello.js")];
+    let mut isolate = Isolate::new(argv, dispatch_sync);
+    tokio_util::init(|| {
+      isolate
+        .execute(
+          "y.js",
+          r#"
+          const m = new Uint8Array([4, 5, 6]);
+          let n = libdeno.send(m);
+          if (!(n.byteLength === 3 &&
+                n[0] === 1 &&
+                n[1] === 2 &&
+                n[2] === 3)) {
+            throw Error("assert error");
+          }
+        "#,
+        ).expect("execute error");
+      isolate.event_loop();
+    });
+  }
+
+  fn dispatch_sync(_state: Arc<IsolateState>, buf: &[u8]) -> (bool, Box<Op>) {
+    assert_eq!(buf[0], 4);
+    assert_eq!(buf[1], 5);
+    assert_eq!(buf[2], 6);
+    // Send back some sync response.
+    let vec: Vec<u8> = vec![1, 2, 3];
+    let buf = vec.into_boxed_slice();
+    let op = Box::new(futures::future::ok(buf));
+    (true, op)
+  }
 }
diff --git a/src/main.rs b/src/main.rs
index 1f9ead0191f0f38a680f2c938cd0a8516a3105e9..b60f8051378051ccd14d0b01d1888a6ef01ae226 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -7,6 +7,7 @@ extern crate msg_rs as msg;
 extern crate rand;
 extern crate tempfile;
 extern crate tokio;
+extern crate tokio_executor;
 extern crate url;
 #[macro_use]
 extern crate lazy_static;
@@ -25,9 +26,9 @@ pub mod handlers;
 mod http;
 mod isolate;
 mod libdeno;
+mod tokio_util;
 mod version;
 
-use isolate::Isolate;
 use std::env;
 
 static LOGGER: Logger = Logger;
@@ -49,18 +50,16 @@ impl log::Log for Logger {
 
 fn main() {
   log::set_logger(&LOGGER).unwrap();
-
   let args = env::args().collect();
-  let mut isolate = Isolate::new(args);
-  flags::process(&isolate.flags);
-
-  isolate
-    .execute("deno_main.js", "denoMain();")
-    .unwrap_or_else(|err| {
-      error!("{}", err);
-      std::process::exit(1);
-    });
-
-  // Start the Tokio event loop
-  isolate.rt.run().expect("err");
+  let mut isolate = isolate::Isolate::new(args, handlers::msg_from_js);
+  flags::process(&isolate.state.flags);
+  tokio_util::init(|| {
+    isolate
+      .execute("deno_main.js", "denoMain();")
+      .unwrap_or_else(|err| {
+        error!("{}", err);
+        std::process::exit(1);
+      });
+    isolate.event_loop();
+  });
 }
diff --git a/src/tokio_util.rs b/src/tokio_util.rs
new file mode 100644
index 0000000000000000000000000000000000000000..de81620ef7fa5d12ec800ca47abc6f9e875934f1
--- /dev/null
+++ b/src/tokio_util.rs
@@ -0,0 +1,30 @@
+// Copyright 2018 the Deno authors. All rights reserved. MIT license.
+
+use futures;
+use futures::Future;
+use tokio;
+use tokio_executor;
+
+pub fn block_on<F, R, E>(future: F) -> Result<R, E>
+where
+  F: Send + 'static + Future<Item = R, Error = E>,
+  R: Send + 'static,
+  E: Send + 'static,
+{
+  let (tx, rx) = futures::sync::oneshot::channel();
+  tokio::spawn(future.then(move |r| tx.send(r).map_err(|_| unreachable!())));
+  rx.wait().unwrap()
+}
+
+// Set the default executor so we can use tokio::spawn(). It's difficult to
+// pass around mut references to the runtime, so using with_default is
+// preferable. Ideally Tokio would provide this function.
+pub fn init<F>(f: F)
+where
+  F: FnOnce(),
+{
+  let rt = tokio::runtime::Runtime::new().unwrap();
+  let mut executor = rt.executor();
+  let mut enter = tokio_executor::enter().expect("Multiple executors at once");
+  tokio_executor::with_default(&mut executor, &mut enter, move |_enter| f());
+}