diff --git a/src/engine/execute.js b/src/engine/execute.js
index 6c58cd156..add9c9909 100644
--- a/src/engine/execute.js
+++ b/src/engine/execute.js
@@ -1,10 +1,19 @@
+var Thread = require('./thread');
+
 /**
  * If set, block calls, args, and return values will be logged to the console.
  * @const {boolean}
  */
 var DEBUG_BLOCK_CALLS = true;
 
-var execute = function (sequencer, thread) {
+/**
+ * Execute a block.
+ * @param {!Sequencer} sequencer Which sequencer is executing.
+ * @param {!Thread} thread Thread which to read and execute.
+ * @param {string=} opt_waitingInputName If evaluating an input, its name.
+ * @return {?Any} Reported value, if available immediately.
+ */
+var execute = function (sequencer, thread, opt_waitingInputName) {
     var runtime = sequencer.runtime;
 
     // Current block to execute is the one on the top of the stack.
@@ -27,11 +36,29 @@ var execute = function (sequencer, thread) {
     for (var inputName in inputs) {
         var input = inputs[inputName];
         var inputBlockId = input.block;
-        // Push to the stack to evaluate this input.
-        thread.pushStack(inputBlockId);
-        var result = execute(sequencer, thread);
-        thread.popStack();
-        argValues[input.name] = result;
+        // Is there a value for this input waiting in the stack frame?
+        if (currentStackFrame.reported &&
+            currentStackFrame.reported[inputName]) {
+            // Use that value.
+            argValues[inputName] = currentStackFrame.reported[inputName];
+        } else {
+            // Otherwise, we need to evaluate the block.
+            // Push to the stack to evaluate this input.
+            thread.pushStack(inputBlockId);
+            if (DEBUG_BLOCK_CALLS) {
+                console.time('Yielding reporter evaluation');
+            }
+            var result = execute(sequencer, thread, inputName);
+            // Did the reporter yield?
+            if (thread.status === Thread.STATUS_YIELD) {
+                // Reporter yielded; don't pop stack and wait for it to unyield.
+                // The value will be populated once the reporter unyields,
+                // and passed up to the currentStackFrame on next execution.
+                return;
+            }
+            thread.popStack();
+            argValues[inputName] = result;
+        }
     }
 
     if (!opcode) {
@@ -51,21 +78,29 @@ var execute = function (sequencer, thread) {
         console.log('and stack frame: ', currentStackFrame);
     }
     var primitiveReturnValue = null;
-    // @todo deal with the return value
     primitiveReturnValue = blockFunction(argValues, {
         yield: thread.yield.bind(thread),
         done: function() {
             sequencer.proceedThread(thread);
         },
+        report: function(reportedValue) {
+            thread.pushReportedValue(opt_waitingInputName, reportedValue);
+            if (DEBUG_BLOCK_CALLS) {
+                console.log('Reported: ', reportedValue,
+                    ' for ', opt_waitingInputName);
+                console.timeEnd('Yielding reporter evaluation');
+            }
+            sequencer.proceedThread(thread);
+        },
         timeout: thread.addTimeout.bind(thread),
-        stackFrame: currentStackFrame,
+        stackFrame: currentStackFrame.executionContext,
         startSubstack: function (substackNum) {
             sequencer.stepToSubstack(thread, substackNum);
         }
     });
     if (DEBUG_BLOCK_CALLS) {
         console.log('ending stack frame: ', currentStackFrame);
-        console.log('returned: ', primitiveReturnValue);
+        console.log('returned immediately: ', primitiveReturnValue);
         console.groupEnd();
     }
     return primitiveReturnValue;
diff --git a/src/engine/sequencer.js b/src/engine/sequencer.js
index 1541d1c6f..e82d4054c 100644
--- a/src/engine/sequencer.js
+++ b/src/engine/sequencer.js
@@ -1,6 +1,5 @@
 var Timer = require('../util/timer');
 var Thread = require('./thread');
-var YieldTimers = require('../util/yieldtimers.js');
 var execute = require('./execute.js');
 
 function Sequencer (runtime) {
@@ -101,7 +100,7 @@ Sequencer.prototype.startThread = function (thread) {
     // move to done.
     if (thread.status === Thread.STATUS_RUNNING &&
         thread.peekStack() === currentBlockId) {
-        this.proceedThread(thread, currentBlockId);
+        this.proceedThread(thread);
     }
 };
 
diff --git a/src/engine/thread.js b/src/engine/thread.js
index bd991db27..65b251648 100644
--- a/src/engine/thread.js
+++ b/src/engine/thread.js
@@ -69,7 +69,10 @@ Thread.prototype.pushStack = function (blockId) {
     // Push an empty stack frame, if we need one.
     // Might not, if we just popped the stack.
     if (this.stack.length > this.stackFrames.length) {
-        this.stackFrames.push({});
+        this.stackFrames.push({
+            reported: {}, // Collects reported input values.
+            executionContext: {} // A context passed to block implementations.
+        });
     }
 };
 
@@ -99,6 +102,18 @@ Thread.prototype.peekStackFrame = function () {
     return this.stackFrames[this.stackFrames.length - 1];
 };
 
+/**
+ * Push a reported value to the parent of the current stack frame.
+ * @param {!string} inputName Name of input reported.
+ * @param {!Any} value Reported value to push.
+ */
+Thread.prototype.pushReportedValue = function (inputName, value) {
+    var parentStackFrame = this.stackFrames[this.stackFrames.length - 2];
+    if (parentStackFrame) {
+        parentStackFrame.reported[inputName] = value;
+    }
+};
+
 /**
  * Yields the thread.
  */