diff --git a/src/wren_core.c b/src/wren_core.c
index 8210b1fc..c7515ed8 100644
--- a/src/wren_core.c
+++ b/src/wren_core.c
@@ -176,6 +176,97 @@ DEF_NATIVE(bool_toString)
   }
 }
 
+DEF_NATIVE(fiber_create)
+{
+  if (!IS_FN(args[1]) && !IS_CLOSURE(args[1]))
+  {
+    RETURN_ERROR("Argument must be a function.");
+  }
+
+  ObjFiber* newFiber = wrenNewFiber(vm, AS_OBJ(args[1]));
+
+  // The compiler expect the first slot of a function to hold the receiver.
+  // Since a fiber's stack is invoked directly, it doesn't have one, so put it
+  // in here.
+  // TODO: Is there a cleaner solution?
+  // TODO: If we make growable stacks, make sure this grows it.
+  newFiber->stack[0] = NULL_VAL;
+  newFiber->stackSize++;
+
+  RETURN_VAL(OBJ_VAL(newFiber));
+}
+
+DEF_NATIVE(fiber_run)
+{
+  // TODO: Error if the fiber is already complete.
+  ObjFiber* runFiber = AS_FIBER(args[0]);
+
+  // Remember who ran it.
+  runFiber->caller = fiber;
+
+  // If the fiber was yielded, make the yield call return null.
+  if (runFiber->stackSize > 0)
+  {
+    runFiber->stack[runFiber->stackSize - 1] = NULL_VAL;
+  }
+
+  return PRIM_RUN_FIBER;
+}
+
+DEF_NATIVE(fiber_run1)
+{
+  // TODO: Error if the fiber is already complete.
+  ObjFiber* runFiber = AS_FIBER(args[0]);
+
+  // Remember who ran it.
+  runFiber->caller = fiber;
+
+  // If the fiber was yielded, make the yield call return the value passed to
+  // run.
+  if (runFiber->stackSize > 0)
+  {
+    runFiber->stack[runFiber->stackSize - 1] = args[1];
+  }
+
+  // When the calling fiber resumes, we'll store the result of the run call
+  // in its stack. Since fiber.run(value) has two arguments (the fiber and the
+  // value) and we only need one slot for the result, discard the other slot
+  // now.
+  fiber->stackSize--;
+
+  return PRIM_RUN_FIBER;
+}
+
+DEF_NATIVE(fiber_yield)
+{
+  // TODO: Handle caller being null.
+
+  // Make the caller's run method return null.
+  fiber->caller->stack[fiber->caller->stackSize - 1] = NULL_VAL;
+
+  // Return the fiber to resume.
+  args[0] = OBJ_VAL(fiber->caller);
+  return PRIM_RUN_FIBER;
+}
+
+DEF_NATIVE(fiber_yield1)
+{
+  // TODO: Handle caller being null.
+
+  // Make the caller's run method return the argument passed to yield.
+  fiber->caller->stack[fiber->caller->stackSize - 1] = args[1];
+
+  // When the yielding fiber resumes, we'll store the result of the yield call
+  // in its stack. Since Fiber.yield(value) has two arguments (the Fiber class
+  // and the value) and we only need one slot for the result, discard the other
+  // slot now.
+  fiber->stackSize--;
+
+  // Return the fiber to resume.
+  args[0] = OBJ_VAL(fiber->caller);
+  return PRIM_RUN_FIBER;
+}
+
 DEF_NATIVE(fn_call) { return PRIM_CALL; }
 
 DEF_NATIVE(list_add)
@@ -603,6 +694,15 @@ void wrenInitializeCore(WrenVM* vm)
   NATIVE(vm->boolClass, "!", bool_not);
 
   vm->fiberClass = defineClass(vm, "Fiber");
+  // TODO: Is there a way we can make this a regular constructor?
+  NATIVE(vm->fiberClass->metaclass, "create ", fiber_create);
+  NATIVE(vm->fiberClass->metaclass, "yield", fiber_yield);
+  NATIVE(vm->fiberClass->metaclass, "yield ", fiber_yield1);
+  NATIVE(vm->fiberClass, "run", fiber_run);
+  NATIVE(vm->fiberClass, "run ", fiber_run1);
+  // TODO: Primitives for switching to a fiber without setting the caller.
+  // (I.e. symmetric coroutines.) Also a getter to tell if a fiber is complete
+  // or not.
 
   vm->fnClass = defineClass(vm, "Function");
   NATIVE(vm->fnClass, "call", fn_call);
diff --git a/src/wren_debug.c b/src/wren_debug.c
index 1421677e..1f2a23a1 100644
--- a/src/wren_debug.c
+++ b/src/wren_debug.c
@@ -350,7 +350,7 @@ void wrenDebugPrintCode(WrenVM* vm, ObjFn* fn)
 
 void wrenDebugPrintStack(ObjFiber* fiber)
 {
-  printf(":: ");
+  printf("(fiber %p) ", fiber);
   for (int i = 0; i < fiber->stackSize; i++)
   {
     wrenPrintValue(fiber->stack[i]);
diff --git a/src/wren_value.c b/src/wren_value.c
index cd5252ec..631abeb3 100644
--- a/src/wren_value.c
+++ b/src/wren_value.c
@@ -129,14 +129,28 @@ ObjClosure* wrenNewClosure(WrenVM* vm, ObjFn* fn)
   return closure;
 }
 
-ObjFiber* wrenNewFiber(WrenVM* vm)
+ObjFiber* wrenNewFiber(WrenVM* vm, Obj* fn)
 {
   ObjFiber* fiber = allocate(vm, sizeof(ObjFiber));
   initObj(vm, &fiber->obj, OBJ_FIBER);
 
+  // Push the stack frame for the function.
   fiber->stackSize = 0;
-  fiber->numFrames = 0;
+  fiber->numFrames = 1;
   fiber->openUpvalues = NULL;
+  fiber->caller = NULL;
+
+  CallFrame* frame = &fiber->frames[0];
+  frame->fn = fn;
+  frame->stackStart = 0;
+  if (fn->type == OBJ_FN)
+  {
+    frame->ip = ((ObjFn*)fn)->bytecode;
+  }
+  else
+  {
+    frame->ip = ((ObjClosure*)fn)->fn->bytecode;
+  }
 
   return fiber;
 }
diff --git a/src/wren_value.h b/src/wren_value.h
index b97759e2..d1141845 100644
--- a/src/wren_value.h
+++ b/src/wren_value.h
@@ -138,7 +138,7 @@ typedef struct
   int stackStart;
 } CallFrame;
 
-typedef struct
+typedef struct sObjFiber
 {
   Obj obj;
   Value stack[STACK_SIZE];
@@ -151,6 +151,10 @@ typedef struct
   // pointing to values still on the stack. The head of the list will be the
   // upvalue closest to the top of the stack, and then the list works downwards.
   Upvalue* openUpvalues;
+
+  // The fiber that ran this one. If this fiber is yielded, control will resume
+  // to this one. May be `NULL`.
+  struct sObjFiber* caller;
 } ObjFiber;
 
 typedef enum
@@ -162,7 +166,11 @@ typedef enum
   PRIM_ERROR,
 
   // A new callframe has been pushed.
-  PRIM_CALL
+  PRIM_CALL,
+
+  // A fiber is being switched to.
+  PRIM_RUN_FIBER
+
 } PrimitiveResult;
 
 typedef struct
@@ -306,6 +314,9 @@ typedef struct
 // Value -> ObjClosure*.
 #define AS_CLOSURE(value) ((ObjClosure*)AS_OBJ(value))
 
+// Value -> ObjFiber*.
+#define AS_FIBER(v) ((ObjFiber*)AS_OBJ(v))
+
 // Value -> ObjFn*.
 #define AS_FN(value) ((ObjFn*)AS_OBJ(value))
 
@@ -500,8 +511,9 @@ void wrenBindMethod(WrenVM* vm, ObjClass* classObj, int symbol, Method method);
 // upvalues, but assumes outside code will populate it.
 ObjClosure* wrenNewClosure(WrenVM* vm, ObjFn* fn);
 
-// Creates a new fiber object.
-ObjFiber* wrenNewFiber(WrenVM* vm);
+// Creates a new fiber object that will invoke [fn], which can be a function or
+// closure.
+ObjFiber* wrenNewFiber(WrenVM* vm, Obj* fn);
 
 // TODO: The argument list here is getting a bit gratuitous.
 // Creates a new function object with the given code and constants. The new
diff --git a/src/wren_vm.c b/src/wren_vm.c
index 07da5372..8e23b750 100644
--- a/src/wren_vm.c
+++ b/src/wren_vm.c
@@ -61,6 +61,7 @@ WrenVM* wrenNewVM(WrenConfiguration* configuration)
   }
 
   vm->compiler = NULL;
+  vm->fiber = NULL;
   vm->first = NULL;
   vm->pinned = NULL;
 
@@ -83,6 +84,8 @@ void wrenFreeVM(WrenVM* vm)
   wrenSymbolTableClear(vm, &vm->methods);
   wrenSymbolTableClear(vm, &vm->globalSymbols);
   wrenReallocate(vm, vm, 0, 0);
+
+  // TODO: Need to free all allocated objects.
 }
 
 static void collectGarbage(WrenVM* vm);
@@ -222,6 +225,9 @@ static void markFiber(WrenVM* vm, ObjFiber* fiber)
     markUpvalue(vm, upvalue);
     upvalue = upvalue->next;
   }
+
+  // The caller.
+  if (fiber->caller != NULL) markFiber(vm, fiber->caller);
 }
 
 static void markClosure(WrenVM* vm, ObjClosure* closure)
@@ -335,6 +341,9 @@ static void collectGarbage(WrenVM* vm)
     pinned = pinned->previous;
   }
 
+  // The current fiber.
+  if (vm->fiber != NULL) wrenMarkObj(vm, (Obj*)vm->fiber);
+
   // Any object the compiler is using (if there is one).
   if (vm->compiler != NULL) wrenMarkCompiler(vm, vm->compiler);
 
@@ -540,8 +549,17 @@ static void callFunction(ObjFiber* fiber, Obj* function, int numArgs)
 // The main bytecode interpreter loop. This is where the magic happens. It is
 // also, as you can imagine, highly performance critical. Returns `true` if the
 // fiber completed without error.
-static bool interpret(WrenVM* vm, ObjFiber* fiber)
+static bool runInterpreter(WrenVM* vm)
 {
+  // Hoist these into local variables. They are accessed frequently in the loop
+  // but assigned less frequently. Keeping them in locals and updating them when
+  // a call frame has been pushed or popped gives a large speed boost.
+  register ObjFiber* fiber = vm->fiber;
+  register CallFrame* frame;
+  register uint8_t* ip;
+  register ObjFn* fn;
+  register Upvalue** upvalues;
+
   // These macros are designed to only be invoked within this function.
   // TODO: Check for stack overflow.
   #define PUSH(value)  (fiber->stack[fiber->stackSize++] = value)
@@ -551,14 +569,6 @@ static bool interpret(WrenVM* vm, ObjFiber* fiber)
   #define READ_BYTE()  (*ip++)
   #define READ_SHORT() (ip += 2, (ip[-2] << 8) | ip[-1])
 
-  // Hoist these into local variables. They are accessed frequently in the loop
-  // but assigned less frequently. Keeping them in locals and updating them when
-  // a call frame has been pushed or popped gives a large speed boost.
-  register CallFrame* frame;
-  register uint8_t* ip;
-  register ObjFn* fn;
-  register Upvalue** upvalues;
-
   // Use this before a CallFrame is pushed to store the local variables back
   // into the current one.
   #define STORE_FRAME() frame->ip = ip
@@ -748,6 +758,12 @@ static bool interpret(WrenVM* vm, ObjFiber* fiber)
               callFunction(fiber, AS_OBJ(args[0]), numArgs);
               LOAD_FRAME();
               break;
+
+            case PRIM_RUN_FIBER:
+              STORE_FRAME();
+              fiber = AS_FIBER(args[0]);
+              LOAD_FRAME();
+              break;
           }
           break;
         }
@@ -848,6 +864,12 @@ static bool interpret(WrenVM* vm, ObjFiber* fiber)
               callFunction(fiber, AS_OBJ(args[0]), numArgs);
               LOAD_FRAME();
               break;
+
+            case PRIM_RUN_FIBER:
+              STORE_FRAME();
+              fiber = AS_FIBER(args[0]);
+              LOAD_FRAME();
+              break;
           }
           break;
         }
@@ -1046,24 +1068,39 @@ static bool interpret(WrenVM* vm, ObjFiber* fiber)
       Value result = POP();
       fiber->numFrames--;
 
-      // If we are returning from the top-level block, we succeeded.
-      if (fiber->numFrames == 0) return true;
-
-      // Close any upvalues still in scope.
-      Value* firstValue = &fiber->stack[frame->stackStart];
-      while (fiber->openUpvalues != NULL &&
-             fiber->openUpvalues->value >= firstValue)
+      // If the fiber is complete, end it.
+      if (fiber->numFrames == 0)
       {
-        closeUpvalue(fiber);
+        // If this is the main fiber, we're done.
+        if (fiber->caller == NULL) return true;
+
+        // TODO: Do we need to close upvalues here?
+
+        // We have a calling fiber to resume.
+        fiber = fiber->caller;
+
+        // Store the result in the resuming fiber.
+        fiber->stack[fiber->stackSize - 1] = result;
+      }
+      else
+      {
+        // Close any upvalues still in scope.
+        Value* firstValue = &fiber->stack[frame->stackStart];
+        while (fiber->openUpvalues != NULL &&
+               fiber->openUpvalues->value >= firstValue)
+        {
+          closeUpvalue(fiber);
+        }
+
+        // Store the result of the block in the first slot, which is where the
+        // caller expects it.
+        fiber->stack[frame->stackStart] = result;
+
+        // Discard the stack slots for the call frame (leaving one slot for the
+        // result).
+        fiber->stackSize = frame->stackStart + 1;
       }
 
-      // Store the result of the block in the first slot, which is where the
-      // caller expects it.
-      fiber->stack[frame->stackStart] = result;
-
-      // Discard the stack slots for the call frame (leaving one slot for the
-      // result).
-      fiber->stackSize = frame->stackStart + 1;
       LOAD_FRAME();
       DISPATCH();
     }
@@ -1168,25 +1205,25 @@ static bool interpret(WrenVM* vm, ObjFiber* fiber)
 
 int wrenInterpret(WrenVM* vm, const char* sourcePath, const char* source)
 {
-  ObjFiber* fiber = wrenNewFiber(vm);
-  PinnedObj pinned;
-  pinObj(vm, (Obj*)fiber, &pinned);
-
   // TODO: Move actual error codes to main.c and return something Wren-specific
   // from here.
   int result = 0;
   ObjFn* fn = wrenCompile(vm, sourcePath, source);
   if (fn != NULL)
   {
-    callFunction(fiber, (Obj*)fn, 0);
-    if (!interpret(vm, fiber)) result = 70; // EX_SOFTWARE.
+    PinnedObj pinned;
+    pinObj(vm, (Obj*)fn, &pinned);
+
+    vm->fiber = wrenNewFiber(vm, (Obj*)fn);
+
+    unpinObj(vm);
+
+    if (!runInterpreter(vm)) result = 70; // EX_SOFTWARE.
   }
   else
   {
     result = 65; // EX_DATAERR.
   }
-
-  unpinObj(vm);
   return result;
 }
 
diff --git a/src/wren_vm.h b/src/wren_vm.h
index 1360c93e..b5c16a44 100644
--- a/src/wren_vm.h
+++ b/src/wren_vm.h
@@ -212,7 +212,10 @@ struct WrenVM
   // allocated objects used by the compiler can be found if a GC is kicked off
   // in the middle of a compile.
   Compiler* compiler;
-  
+
+  // The fiber that is currently running.
+  ObjFiber* fiber;
+
   // Memory management data:
 
   // The number of bytes that are known to be currently allocated. Includes all
diff --git a/test/fiber/resume_caller.wren b/test/fiber/resume_caller.wren
new file mode 100644
index 00000000..502e9bca
--- /dev/null
+++ b/test/fiber/resume_caller.wren
@@ -0,0 +1,19 @@
+var b = Fiber.create(fn {
+  IO.print("fiber b")
+})
+
+var a = Fiber.create(fn {
+  IO.print("begin fiber a")
+  b.run
+  IO.print("end fiber a")
+})
+
+IO.print("begin main")
+a.run
+IO.print("end main")
+
+// expect: begin main
+// expect: begin fiber a
+// expect: fiber b
+// expect: end fiber a
+// expect: end main
diff --git a/test/fiber/run.wren b/test/fiber/run.wren
new file mode 100644
index 00000000..fd54fd9f
--- /dev/null
+++ b/test/fiber/run.wren
@@ -0,0 +1,12 @@
+var fiber = Fiber.create(fn {
+  IO.print("fiber")
+})
+
+IO.print("before") // expect: before
+fiber.run          // expect: fiber
+IO.print("after")  // expect: after
+
+// TODO: Test handles error if fiber tries to run itself.
+// TODO: Test create is passed right argument type.
+// TODO: Test closing over stuff in fiber function.
+// TODO: Test running a finished fiber.
diff --git a/test/fiber/run_return_implicit_null.wren b/test/fiber/run_return_implicit_null.wren
new file mode 100644
index 00000000..04616807
--- /dev/null
+++ b/test/fiber/run_return_implicit_null.wren
@@ -0,0 +1,6 @@
+var fiber = Fiber.create(fn {
+  IO.print("fiber")
+})
+
+var result = fiber.run // expect: fiber
+IO.print(result)       // expect: null
diff --git a/test/fiber/run_return_value.wren b/test/fiber/run_return_value.wren
new file mode 100644
index 00000000..5414019e
--- /dev/null
+++ b/test/fiber/run_return_value.wren
@@ -0,0 +1,7 @@
+var fiber = Fiber.create(fn {
+  IO.print("fiber")
+  return "result"
+})
+
+var result = fiber.run // expect: fiber
+IO.print(result)       // expect: result
diff --git a/test/fiber/run_with_value.wren b/test/fiber/run_with_value.wren
new file mode 100644
index 00000000..d41315be
--- /dev/null
+++ b/test/fiber/run_with_value.wren
@@ -0,0 +1,9 @@
+var fiber = Fiber.create(fn {
+  IO.print("fiber")
+})
+
+// The first value passed to the fiber is ignored, since there's no yield call
+// to return it.
+IO.print("before")   // expect: before
+fiber.run("ignored") // expect: fiber
+IO.print("after")    // expect: after
diff --git a/test/fiber/type.wren b/test/fiber/type.wren
new file mode 100644
index 00000000..14ecb594
--- /dev/null
+++ b/test/fiber/type.wren
@@ -0,0 +1,5 @@
+var fiber = Fiber.create(fn null)
+IO.print(fiber is Fiber)      // expect: true
+IO.print(fiber is Object)     // expect: true
+IO.print(fiber is Bool)       // expect: false
+IO.print(fiber.type == Fiber) // expect: true
diff --git a/test/fiber/yield.wren b/test/fiber/yield.wren
new file mode 100644
index 00000000..ef8092db
--- /dev/null
+++ b/test/fiber/yield.wren
@@ -0,0 +1,14 @@
+var fiber = Fiber.create(fn {
+  IO.print("fiber 1")
+  Fiber.yield
+  IO.print("fiber 2")
+  Fiber.yield
+  IO.print("fiber 3")
+})
+
+var result = fiber.run // expect: fiber 1
+IO.print("main 1") // expect: main 1
+result = fiber.run // expect: fiber 2
+IO.print("main 2") // expect: main 2
+result = fiber.run // expect: fiber 3
+IO.print("main 3") // expect: main 3
diff --git a/test/fiber/yield_return_value.wren b/test/fiber/yield_return_value.wren
new file mode 100644
index 00000000..cb03725a
--- /dev/null
+++ b/test/fiber/yield_return_value.wren
@@ -0,0 +1,14 @@
+var fiber = Fiber.create(fn {
+  IO.print("fiber 1")
+  var result = Fiber.yield
+  IO.print(result)
+  result = Fiber.yield
+  IO.print(result)
+})
+
+fiber.run // expect: fiber 1
+IO.print("main 1") // expect: main 1
+fiber.run("run 1") // expect: run 1
+IO.print("main 2") // expect: main 2
+fiber.run // expect: null
+IO.print("main 3") // expect: main 3
diff --git a/test/fiber/yield_with_value.wren b/test/fiber/yield_with_value.wren
new file mode 100644
index 00000000..14198476
--- /dev/null
+++ b/test/fiber/yield_with_value.wren
@@ -0,0 +1,14 @@
+var fiber = Fiber.create(fn {
+  IO.print("fiber 1")
+  Fiber.yield("yield 1")
+  IO.print("fiber 2")
+  Fiber.yield("yield 2")
+  IO.print("fiber 3")
+})
+
+var result = fiber.run // expect: fiber 1
+IO.print(result) // expect: yield 1
+result = fiber.run // expect: fiber 2
+IO.print(result) // expect: yield 2
+result = fiber.run // expect: fiber 3
+IO.print(result) // expect: null