diff --git a/extmod/uasyncio/__init__.py b/extmod/uasyncio/__init__.py
new file mode 100644
index 0000000000..6bff13883a
--- /dev/null
+++ b/extmod/uasyncio/__init__.py
@@ -0,0 +1,26 @@
+# MicroPython uasyncio module
+# MIT license; Copyright (c) 2019 Damien P. George
+
+from .core import *
+
+__version__ = (3, 0, 0)
+
+_attrs = {
+    "wait_for": "funcs",
+    "gather": "funcs",
+    "Event": "event",
+    "Lock": "lock",
+    "open_connection": "stream",
+    "start_server": "stream",
+}
+
+# Lazy loader, effectively does:
+#   global attr
+#   from .mod import attr
+def __getattr__(attr):
+    mod = _attrs.get(attr, None)
+    if mod is None:
+        raise AttributeError(attr)
+    value = getattr(__import__(mod, None, None, True, 1), attr)
+    globals()[attr] = value
+    return value
diff --git a/extmod/uasyncio/core.py b/extmod/uasyncio/core.py
new file mode 100644
index 0000000000..049e2537f8
--- /dev/null
+++ b/extmod/uasyncio/core.py
@@ -0,0 +1,231 @@
+# MicroPython uasyncio module
+# MIT license; Copyright (c) 2019 Damien P. George
+
+from time import ticks_ms as ticks, ticks_diff, ticks_add
+import sys, select
+
+# Import TaskQueue and Task
+from .task import TaskQueue, Task
+
+
+################################################################################
+# Exceptions
+
+
+class CancelledError(BaseException):
+    pass
+
+
+class TimeoutError(Exception):
+    pass
+
+
+################################################################################
+# Sleep functions
+
+# "Yield" once, then raise StopIteration
+class SingletonGenerator:
+    def __init__(self):
+        self.state = None
+        self.exc = StopIteration()
+
+    def __iter__(self):
+        return self
+
+    def __next__(self):
+        if self.state is not None:
+            _task_queue.push_sorted(cur_task, self.state)
+            self.state = None
+            return None
+        else:
+            self.exc.__traceback__ = None
+            raise self.exc
+
+
+# Pause task execution for the given time (integer in milliseconds, uPy extension)
+# Use a SingletonGenerator to do it without allocating on the heap
+def sleep_ms(t, sgen=SingletonGenerator()):
+    assert sgen.state is None
+    sgen.state = ticks_add(ticks(), t)
+    return sgen
+
+
+# Pause task execution for the given time (in seconds)
+def sleep(t):
+    return sleep_ms(int(t * 1000))
+
+
+################################################################################
+# Queue and poller for stream IO
+
+
+class IOQueue:
+    def __init__(self):
+        self.poller = select.poll()
+        self.map = {}  # maps id(stream) to [task_waiting_read, task_waiting_write, stream]
+
+    def _enqueue(self, s, idx):
+        if id(s) not in self.map:
+            entry = [None, None, s]
+            entry[idx] = cur_task
+            self.map[id(s)] = entry
+            self.poller.register(s, select.POLLIN if idx == 0 else select.POLLOUT)
+        else:
+            sm = self.map[id(s)]
+            assert sm[idx] is None
+            assert sm[1 - idx] is not None
+            sm[idx] = cur_task
+            self.poller.modify(s, select.POLLIN | select.POLLOUT)
+        # Link task to this IOQueue so it can be removed if needed
+        cur_task.data = self
+
+    def _dequeue(self, s):
+        del self.map[id(s)]
+        self.poller.unregister(s)
+
+    def queue_read(self, s):
+        self._enqueue(s, 0)
+
+    def queue_write(self, s):
+        self._enqueue(s, 1)
+
+    def remove(self, task):
+        while True:
+            del_s = None
+            for k in self.map:  # Iterate without allocating on the heap
+                q0, q1, s = self.map[k]
+                if q0 is task or q1 is task:
+                    del_s = s
+                    break
+            if del_s is not None:
+                self._dequeue(s)
+            else:
+                break
+
+    def wait_io_event(self, dt):
+        for s, ev in self.poller.ipoll(dt):
+            sm = self.map[id(s)]
+            # print('poll', s, sm, ev)
+            if ev & ~select.POLLOUT and sm[0] is not None:
+                # POLLIN or error
+                _task_queue.push_head(sm[0])
+                sm[0] = None
+            if ev & ~select.POLLIN and sm[1] is not None:
+                # POLLOUT or error
+                _task_queue.push_head(sm[1])
+                sm[1] = None
+            if sm[0] is None and sm[1] is None:
+                self._dequeue(s)
+            elif sm[0] is None:
+                self.poller.modify(s, select.POLLOUT)
+            else:
+                self.poller.modify(s, select.POLLIN)
+
+
+################################################################################
+# Main run loop
+
+# TaskQueue of Task instances
+_task_queue = TaskQueue()
+
+# Task queue and poller for stream IO
+_io_queue = IOQueue()
+
+
+# Ensure the awaitable is a task
+def _promote_to_task(aw):
+    return aw if isinstance(aw, Task) else create_task(aw)
+
+
+# Create and schedule a new task from a coroutine
+def create_task(coro):
+    if not hasattr(coro, "send"):
+        raise TypeError("coroutine expected")
+    t = Task(coro, globals())
+    _task_queue.push_head(t)
+    return t
+
+
+# Keep scheduling tasks until there are none left to schedule
+def run_until_complete(main_task=None):
+    global cur_task
+    excs_all = (CancelledError, Exception)  # To prevent heap allocation in loop
+    excs_stop = (CancelledError, StopIteration)  # To prevent heap allocation in loop
+    while True:
+        # Wait until the head of _task_queue is ready to run
+        dt = 1
+        while dt > 0:
+            dt = -1
+            t = _task_queue.peek()
+            if t:
+                # A task waiting on _task_queue; "ph_key" is time to schedule task at
+                dt = max(0, ticks_diff(t.ph_key, ticks()))
+            elif not _io_queue.map:
+                # No tasks can be woken so finished running
+                return
+            # print('(poll {})'.format(dt), len(_io_queue.map))
+            _io_queue.wait_io_event(dt)
+
+        # Get next task to run and continue it
+        t = _task_queue.pop_head()
+        cur_task = t
+        try:
+            # Continue running the coroutine, it's responsible for rescheduling itself
+            exc = t.data
+            if not exc:
+                t.coro.send(None)
+            else:
+                t.data = None
+                t.coro.throw(exc)
+        except excs_all as er:
+            # Check the task is not on any event queue
+            assert t.data is None
+            # This task is done, check if it's the main task and then loop should stop
+            if t is main_task:
+                if isinstance(er, StopIteration):
+                    return er.value
+                raise er
+            # Save return value of coro to pass up to caller
+            t.data = er
+            # Schedule any other tasks waiting on the completion of this task
+            waiting = False
+            if hasattr(t, "waiting"):
+                while t.waiting.peek():
+                    _task_queue.push_head(t.waiting.pop_head())
+                    waiting = True
+                t.waiting = None  # Free waiting queue head
+            # Print out exception for detached tasks
+            if not waiting and not isinstance(er, excs_stop):
+                print("task raised exception:", t.coro)
+                sys.print_exception(er)
+            # Indicate task is done
+            t.coro = None
+
+
+# Create a new task from a coroutine and run it until it finishes
+def run(coro):
+    return run_until_complete(create_task(coro))
+
+
+################################################################################
+# Event loop wrapper
+
+
+class Loop:
+    def create_task(self, coro):
+        return create_task(coro)
+
+    def run_forever(self):
+        run_until_complete()
+        # TODO should keep running until .stop() is called, even if there're no tasks left
+
+    def run_until_complete(self, aw):
+        return run_until_complete(_promote_to_task(aw))
+
+    def close(self):
+        pass
+
+
+# The runq_len and waitq_len arguments are for legacy uasyncio compatibility
+def get_event_loop(runq_len=0, waitq_len=0):
+    return Loop()
diff --git a/extmod/uasyncio/event.py b/extmod/uasyncio/event.py
new file mode 100644
index 0000000000..31cb00e055
--- /dev/null
+++ b/extmod/uasyncio/event.py
@@ -0,0 +1,31 @@
+# MicroPython uasyncio module
+# MIT license; Copyright (c) 2019-2020 Damien P. George
+
+from . import core
+
+# Event class for primitive events that can be waited on, set, and cleared
+class Event:
+    def __init__(self):
+        self.state = False  # False=unset; True=set
+        self.waiting = core.TaskQueue()  # Queue of Tasks waiting on completion of this event
+
+    def is_set(self):
+        return self.state
+
+    def set(self):
+        # Event becomes set, schedule any tasks waiting on it
+        while self.waiting.peek():
+            core._task_queue.push_head(self.waiting.pop_head())
+        self.state = True
+
+    def clear(self):
+        self.state = False
+
+    async def wait(self):
+        if not self.state:
+            # Event not set, put the calling task on the event's waiting queue
+            self.waiting.push_head(core.cur_task)
+            # Set calling task's data to the event's queue so it can be removed if needed
+            core.cur_task.data = self.waiting
+            yield
+        return True
diff --git a/extmod/uasyncio/funcs.py b/extmod/uasyncio/funcs.py
new file mode 100644
index 0000000000..7a4bddf256
--- /dev/null
+++ b/extmod/uasyncio/funcs.py
@@ -0,0 +1,50 @@
+# MicroPython uasyncio module
+# MIT license; Copyright (c) 2019-2020 Damien P. George
+
+from . import core
+
+
+async def wait_for(aw, timeout):
+    aw = core._promote_to_task(aw)
+    if timeout is None:
+        return await aw
+
+    def cancel(aw, timeout):
+        await core.sleep(timeout)
+        aw.cancel()
+
+    cancel_task = core.create_task(cancel(aw, timeout))
+    try:
+        ret = await aw
+    except core.CancelledError:
+        # Ignore CancelledError from aw, it's probably due to timeout
+        pass
+    finally:
+        # Cancel the "cancel" task if it's still active (optimisation instead of cancel_task.cancel())
+        if cancel_task.coro is not None:
+            core._task_queue.remove(cancel_task)
+    if cancel_task.coro is None:
+        # Cancel task ran to completion, ie there was a timeout
+        raise core.TimeoutError
+    return ret
+
+
+async def gather(*aws, return_exceptions=False):
+    ts = [core._promote_to_task(aw) for aw in aws]
+    for i in range(len(ts)):
+        try:
+            # TODO handle cancel of gather itself
+            # if ts[i].coro:
+            #    iter(ts[i]).waiting.push_head(cur_task)
+            #    try:
+            #        yield
+            #    except CancelledError as er:
+            #        # cancel all waiting tasks
+            #        raise er
+            ts[i] = await ts[i]
+        except Exception as er:
+            if return_exceptions:
+                ts[i] = er
+            else:
+                raise er
+    return ts
diff --git a/extmod/uasyncio/lock.py b/extmod/uasyncio/lock.py
new file mode 100644
index 0000000000..18a55cb483
--- /dev/null
+++ b/extmod/uasyncio/lock.py
@@ -0,0 +1,53 @@
+# MicroPython uasyncio module
+# MIT license; Copyright (c) 2019-2020 Damien P. George
+
+from . import core
+
+# Lock class for primitive mutex capability
+class Lock:
+    def __init__(self):
+        # The state can take the following values:
+        # - 0: unlocked
+        # - 1: locked
+        # - <Task>: unlocked but this task has been scheduled to acquire the lock next
+        self.state = 0
+        # Queue of Tasks waiting to acquire this Lock
+        self.waiting = core.TaskQueue()
+
+    def locked(self):
+        return self.state == 1
+
+    def release(self):
+        if self.state != 1:
+            raise RuntimeError
+        if self.waiting.peek():
+            # Task(s) waiting on lock, schedule next Task
+            self.state = self.waiting.pop_head()
+            core._task_queue.push_head(self.state)
+        else:
+            # No Task waiting so unlock
+            self.state = 0
+
+    async def acquire(self):
+        if self.state != 0:
+            # Lock unavailable, put the calling Task on the waiting queue
+            self.waiting.push_head(core.cur_task)
+            # Set calling task's data to the lock's queue so it can be removed if needed
+            core.cur_task.data = self.waiting
+            try:
+                yield
+            except core.CancelledError as er:
+                if self.state == core.cur_task:
+                    # Cancelled while pending on resume, schedule next waiting Task
+                    self.state = 1
+                    self.release()
+                raise er
+        # Lock available, set it as locked
+        self.state = 1
+        return True
+
+    async def __aenter__(self):
+        return await self.acquire()
+
+    async def __aexit__(self, exc_type, exc, tb):
+        return self.release()
diff --git a/extmod/uasyncio/stream.py b/extmod/uasyncio/stream.py
new file mode 100644
index 0000000000..7803ac4bfa
--- /dev/null
+++ b/extmod/uasyncio/stream.py
@@ -0,0 +1,141 @@
+# MicroPython uasyncio module
+# MIT license; Copyright (c) 2019-2020 Damien P. George
+
+from . import core
+
+
+class Stream:
+    def __init__(self, s, e={}):
+        self.s = s
+        self.e = e
+        self.out_buf = b""
+
+    def get_extra_info(self, v):
+        return self.e[v]
+
+    async def __aenter__(self):
+        return self
+
+    async def __aexit__(self, exc_type, exc, tb):
+        await self.close()
+
+    def close(self):
+        pass
+
+    async def wait_closed(self):
+        # TODO yield?
+        self.s.close()
+
+    async def read(self, n):
+        yield core._io_queue.queue_read(self.s)
+        return self.s.read(n)
+
+    async def readline(self):
+        l = b""
+        while True:
+            yield core._io_queue.queue_read(self.s)
+            l2 = self.s.readline()  # may do multiple reads but won't block
+            l += l2
+            if not l2 or l[-1] == 10:  # \n (check l in case l2 is str)
+                return l
+
+    def write(self, buf):
+        self.out_buf += buf
+
+    async def drain(self):
+        mv = memoryview(self.out_buf)
+        off = 0
+        while off < len(mv):
+            yield core._io_queue.queue_write(self.s)
+            ret = self.s.write(mv[off:])
+            if ret is not None:
+                off += ret
+        self.out_buf = b""
+
+
+# Create a TCP stream connection to a remote host
+async def open_connection(host, port):
+    from uerrno import EINPROGRESS
+    import usocket as socket
+
+    ai = socket.getaddrinfo(host, port)[0]  # TODO this is blocking!
+    s = socket.socket()
+    s.setblocking(False)
+    ss = Stream(s)
+    try:
+        s.connect(ai[-1])
+    except OSError as er:
+        if er.args[0] != EINPROGRESS:
+            raise er
+    yield core._io_queue.queue_write(s)
+    return ss, ss
+
+
+# Class representing a TCP stream server, can be closed and used in "async with"
+class Server:
+    async def __aenter__(self):
+        return self
+
+    async def __aexit__(self, exc_type, exc, tb):
+        self.close()
+        await self.wait_closed()
+
+    def close(self):
+        self.task.cancel()
+
+    async def wait_closed(self):
+        await self.task
+
+    async def _serve(self, cb, host, port, backlog):
+        import usocket as socket
+
+        ai = socket.getaddrinfo(host, port)[0]  # TODO this is blocking!
+        s = socket.socket()
+        s.setblocking(False)
+        s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+        s.bind(ai[-1])
+        s.listen(backlog)
+        self.task = core.cur_task
+        # Accept incoming connections
+        while True:
+            try:
+                yield core._io_queue.queue_read(s)
+            except core.CancelledError:
+                # Shutdown server
+                s.close()
+                return
+            try:
+                s2, addr = s.accept()
+            except:
+                # Ignore a failed accept
+                continue
+            s2.setblocking(False)
+            s2s = Stream(s2, {"peername": addr})
+            core.create_task(cb(s2s, s2s))
+
+
+# Helper function to start a TCP stream server, running as a new task
+# TODO could use an accept-callback on socket read activity instead of creating a task
+async def start_server(cb, host, port, backlog=5):
+    s = Server()
+    core.create_task(s._serve(cb, host, port, backlog))
+    return s
+
+
+################################################################################
+# Legacy uasyncio compatibility
+
+
+async def stream_awrite(self, buf, off=0, sz=-1):
+    if off != 0 or sz != -1:
+        buf = memoryview(buf)
+        if sz == -1:
+            sz = len(buf)
+        buf = buf[off : off + sz]
+    self.write(buf)
+    await self.drain()
+
+
+Stream.aclose = Stream.wait_closed
+Stream.awrite = stream_awrite
+Stream.awritestr = stream_awrite  # TODO explicitly convert to bytes?
diff --git a/extmod/uasyncio/task.py b/extmod/uasyncio/task.py
new file mode 100644
index 0000000000..1d5bcc5cf7
--- /dev/null
+++ b/extmod/uasyncio/task.py
@@ -0,0 +1,168 @@
+# MicroPython uasyncio module
+# MIT license; Copyright (c) 2019-2020 Damien P. George
+
+# This file contains the core TaskQueue based on a pairing heap, and the core Task class.
+# They can optionally be replaced by C implementations.
+
+from . import core
+
+
+# pairing-heap meld of 2 heaps; O(1)
+def ph_meld(h1, h2):
+    if h1 is None:
+        return h2
+    if h2 is None:
+        return h1
+    lt = core.ticks_diff(h1.ph_key, h2.ph_key) < 0
+    if lt:
+        if h1.ph_child is None:
+            h1.ph_child = h2
+        else:
+            h1.ph_child_last.ph_next = h2
+        h1.ph_child_last = h2
+        h2.ph_next = None
+        h2.ph_rightmost_parent = h1
+        return h1
+    else:
+        h1.ph_next = h2.ph_child
+        h2.ph_child = h1
+        if h1.ph_next is None:
+            h2.ph_child_last = h1
+            h1.ph_rightmost_parent = h2
+        return h2
+
+
+# pairing-heap pairing operation; amortised O(log N)
+def ph_pairing(child):
+    heap = None
+    while child is not None:
+        n1 = child
+        child = child.ph_next
+        n1.ph_next = None
+        if child is not None:
+            n2 = child
+            child = child.ph_next
+            n2.ph_next = None
+            n1 = ph_meld(n1, n2)
+        heap = ph_meld(heap, n1)
+    return heap
+
+
+# pairing-heap delete of a node; stable, amortised O(log N)
+def ph_delete(heap, node):
+    if node is heap:
+        child = heap.ph_child
+        node.ph_child = None
+        return ph_pairing(child)
+    # Find parent of node
+    parent = node
+    while parent.ph_next is not None:
+        parent = parent.ph_next
+    parent = parent.ph_rightmost_parent
+    # Replace node with pairing of its children
+    if node is parent.ph_child and node.ph_child is None:
+        parent.ph_child = node.ph_next
+        node.ph_next = None
+        return heap
+    elif node is parent.ph_child:
+        child = node.ph_child
+        next = node.ph_next
+        node.ph_child = None
+        node.ph_next = None
+        node = ph_pairing(child)
+        parent.ph_child = node
+    else:
+        n = parent.ph_child
+        while node is not n.ph_next:
+            n = n.ph_next
+        child = node.ph_child
+        next = node.ph_next
+        node.ph_child = None
+        node.ph_next = None
+        node = ph_pairing(child)
+        if node is None:
+            node = n
+        else:
+            n.ph_next = node
+    node.ph_next = next
+    if next is None:
+        node.ph_rightmost_parent = parent
+        parent.ph_child_last = node
+    return heap
+
+
+# TaskQueue class based on the above pairing-heap functions.
+class TaskQueue:
+    def __init__(self):
+        self.heap = None
+
+    def peek(self):
+        return self.heap
+
+    def push_sorted(self, v, key):
+        v.data = None
+        v.ph_key = key
+        v.ph_child = None
+        v.ph_next = None
+        self.heap = ph_meld(v, self.heap)
+
+    def push_head(self, v):
+        self.push_sorted(v, core.ticks())
+
+    def pop_head(self):
+        v = self.heap
+        self.heap = ph_pairing(self.heap.ph_child)
+        return v
+
+    def remove(self, v):
+        self.heap = ph_delete(self.heap, v)
+
+
+# Task class representing a coroutine, can be waited on and cancelled.
+class Task:
+    def __init__(self, coro, globals=None):
+        self.coro = coro  # Coroutine of this Task
+        self.data = None  # General data for queue it is waiting on
+        self.ph_key = 0  # Pairing heap
+        self.ph_child = None  # Paring heap
+        self.ph_child_last = None  # Paring heap
+        self.ph_next = None  # Paring heap
+        self.ph_rightmost_parent = None  # Paring heap
+
+    def __iter__(self):
+        if not hasattr(self, "waiting"):
+            # Lazily allocated head of linked list of Tasks waiting on completion of this task.
+            self.waiting = TaskQueue()
+        return self
+
+    def __next__(self):
+        if not self.coro:
+            # Task finished, raise return value to caller so it can continue.
+            raise self.data
+        else:
+            # Put calling task on waiting queue.
+            self.waiting.push_head(core.cur_task)
+            # Set calling task's data to this task that it waits on, to double-link it.
+            core.cur_task.data = self
+
+    def cancel(self):
+        # Check if task is already finished.
+        if self.coro is None:
+            return False
+        # Can't cancel self (not supported yet).
+        if self is core.cur_task:
+            raise RuntimeError("cannot cancel self")
+        # If Task waits on another task then forward the cancel to the one it's waiting on.
+        while isinstance(self.data, Task):
+            self = self.data
+        # Reschedule Task as a cancelled task.
+        if hasattr(self.data, "remove"):
+            # Not on the main running queue, remove the task from the queue it's on.
+            self.data.remove(self)
+            core._task_queue.push_head(self)
+        elif core.ticks_diff(self.ph_key, core.ticks()) > 0:
+            # On the main running queue but scheduled in the future, so bring it forward to now.
+            core._task_queue.remove(self)
+            core._task_queue.push_head(self)
+        self.data = core.CancelledError
+        return True