diff --git a/lib/python/ray/worker.py b/lib/python/ray/worker.py
index edbab95a5..9c184eacd 100644
--- a/lib/python/ray/worker.py
+++ b/lib/python/ray/worker.py
@@ -466,6 +466,7 @@ class Worker(object):
         be object IDs or they can be values. If they are values, they
         must be serializable objecs.
     """
+    check_main_thread()
     # Put large or complex arguments that are passed by value in the object
     # store first.
     args_for_photon = []
@@ -503,6 +504,7 @@ class Worker(object):
         not take any arguments. If it returns anything, its return values will
         not be used.
     """
+    check_main_thread()
     if self.mode not in [None, SCRIPT_MODE, SILENT_MODE, PYTHON_MODE]:
       raise Exception("run_function_on_all_workers can only be called on a driver.")
     # If ray.init has not been called yet, then cache the function and export it
@@ -540,6 +542,16 @@ made by one task do not affect other tasks.
 class RayConnectionError(Exception):
   pass
 
+def check_main_thread():
+  """Check that we are currently on the main thread.
+
+  Raises:
+    Exception: An exception is raised if this is called on a thread other than
+      the main thread.
+  """
+  if threading.current_thread().getName() != "MainThread":
+    raise Exception("The Ray methods are not thread safe and must be called from the main thread. This method was called from thread {}.".format(threading.current_thread().getName()))
+
 def check_connected(worker=global_worker):
   """Check if the worker is connected.
 
@@ -565,6 +577,7 @@ def print_failed_task(task_status):
 
 def error_info(worker=global_worker):
   """Return information about failed tasks."""
+  check_main_thread()
   check_connected(worker)
   result = {"TaskError": [],
             "RemoteFunctionImportError": [],
@@ -627,6 +640,7 @@ def init(start_ray_local=False, num_workers=None, num_local_schedulers=1, driver
     Exception: An exception is raised if an inappropriate combination of
       arguments is passed in.
   """
+  check_main_thread()
   if driver_mode == PYTHON_MODE:
     # If starting Ray in PYTHON_MODE, don't start any other processes.
     address_info = {}
@@ -819,6 +833,7 @@ def connect(address_info, mode=WORKER_MODE, worker=global_worker):
     mode: The mode of the worker. One of SCRIPT_MODE, WORKER_MODE, PYTHON_MODE,
       and SILENT_MODE.
   """
+  check_main_thread()
   worker.worker_id = random_string()
   worker.connected = True
   worker.set_mode(mode)
@@ -952,6 +967,7 @@ def get(objectid, worker=global_worker):
   Returns:
     A Python object or a list of Python objects.
   """
+  check_main_thread()
   check_connected(worker)
   if worker.mode == PYTHON_MODE:
     return objectid # In PYTHON_MODE, ray.get is the identity operation (the input will actually be a value not an objectid)
@@ -977,6 +993,7 @@ def put(value, worker=global_worker):
   Returns:
     The object ID assigned to this value.
   """
+  check_main_thread()
   check_connected(worker)
   if worker.mode == PYTHON_MODE:
     return value # In PYTHON_MODE, ray.put is the identity operation
@@ -1006,6 +1023,7 @@ def wait(object_ids, num_returns=1, timeout=None, worker=global_worker):
   Returns:
     A list of object IDs that are ready and a list of the remaining object IDs.
   """
+  check_main_thread()
   check_connected(worker)
   object_id_strs = [object_id.id() for object_id in object_ids]
   timeout = timeout if timeout is not None else 2 ** 30
@@ -1095,6 +1113,7 @@ def main_loop(worker=global_worker):
                                             "message": traceback_str})
       worker.redis_client.rpush("ErrorKeys", error_key)
 
+  check_main_thread()
   while True:
     task = worker.photon_client.get_task()
     function_id = task.function_id()
@@ -1145,6 +1164,7 @@ def _export_reusable_variable(name, reusable, worker=global_worker):
     reusable (Reusable): The reusable object containing code for initializing
       and reinitializing the variable.
   """
+  check_main_thread()
   if _mode(worker) not in [SCRIPT_MODE, SILENT_MODE]:
     raise Exception("_export_reusable_variable can only be called on a driver.")
   reusable_variable_id = name
@@ -1156,6 +1176,7 @@ def _export_reusable_variable(name, reusable, worker=global_worker):
   worker.driver_export_counter += 1
 
 def export_remote_function(function_id, func_name, func, num_return_vals, worker=global_worker):
+  check_main_thread()
   if _mode(worker) not in [SCRIPT_MODE, SILENT_MODE]:
     raise Exception("export_remote_function can only be called on a driver.")
   key = "RemoteFunction:{}".format(function_id.id())
@@ -1188,6 +1209,7 @@ def remote(*args, **kwargs):
 
       def func_call(*args, **kwargs):
         """This gets run immediately when a worker calls a remote function."""
+        check_main_thread()
         check_connected()
         args = list(args)
         args.extend([kwargs[keyword] if kwargs.has_key(keyword) else default for keyword, default in keyword_defaults[len(args):]]) # fill in the remaining arguments