diff --git a/playground/index.html b/playground/index.html
index bbfc80324..e8b980141 100644
--- a/playground/index.html
+++ b/playground/index.html
@@ -404,7 +404,7 @@
       <category name="Data" colour="#FF8C1A" custom="VARIABLE">
       </category>
       <category name="Lists" colour="#FF8C1A">
-        <block type="data_list"></block>
+        <block type="data_listcontents"></block>
         <block type="data_addtolist">
           <value name="ITEM">
             <shadow type="text">
diff --git a/src/blocks/scratch3_procedures.js b/src/blocks/scratch3_procedures.js
new file mode 100644
index 000000000..8d43c4c92
--- /dev/null
+++ b/src/blocks/scratch3_procedures.js
@@ -0,0 +1,32 @@
+function Scratch3ProcedureBlocks(runtime) {
+    /**
+     * The runtime instantiating this block package.
+     * @type {Runtime}
+     */
+    this.runtime = runtime;
+}
+
+/**
+ * Retrieve the block primitives implemented by this package.
+ * @return {Object.<string, Function>} Mapping of opcode to Function.
+ */
+Scratch3ProcedureBlocks.prototype.getPrimitives = function() {
+    return {
+        'procedures_defnoreturn': this.defNoReturn,
+        'procedures_callnoreturn': this.callNoReturn
+    };
+};
+
+Scratch3ProcedureBlocks.prototype.defNoReturn = function () {
+    // No-op: execute the blocks.
+};
+
+Scratch3ProcedureBlocks.prototype.callNoReturn = function (args, util) {
+    if (!util.stackFrame.executed) {
+        var procedureName = args.mutation.name;
+        util.stackFrame.executed = true;
+        util.startProcedure(procedureName);
+    }
+};
+
+module.exports = Scratch3ProcedureBlocks;
diff --git a/src/engine/adapter.js b/src/engine/adapter.js
index 26f52be90..327aae5e3 100644
--- a/src/engine/adapter.js
+++ b/src/engine/adapter.js
@@ -1,3 +1,4 @@
+var mutationAdapter = require('./mutation-adapter');
 var html = require('htmlparser2');
 
 /**
@@ -138,6 +139,9 @@ function domToBlock (blockDOM, blocks, isTopBlock, parent) {
             // Link next block to this block.
             block.next = childBlockNode.attribs.id;
             break;
+        case 'mutation':
+            block.mutation = mutationAdapter(xmlChild);
+            break;
         }
     }
 }
diff --git a/src/engine/blocks.js b/src/engine/blocks.js
index 99f52477d..f96c916ad 100644
--- a/src/engine/blocks.js
+++ b/src/engine/blocks.js
@@ -1,4 +1,5 @@
 var adapter = require('./adapter');
+var mutationAdapter = require('./mutation-adapter');
 var xmlEscape = require('../util/xml-escape');
 
 /**
@@ -116,6 +117,16 @@ Blocks.prototype.getInputs = function (id) {
     return inputs;
 };
 
+/**
+ * Get mutation data for a block.
+ * @param {?string} id ID of block to query.
+ * @return {!Object} Mutation for the block.
+ */
+Blocks.prototype.getMutation = function (id) {
+    if (typeof this._blocks[id] === 'undefined') return null;
+    return this._blocks[id].mutation;
+};
+
 /**
  * Get the top-level script for a given block.
  * @param {?string} id ID of block to query.
@@ -130,6 +141,23 @@ Blocks.prototype.getTopLevelScript = function (id) {
     return block.id;
 };
 
+/**
+ * Get the procedure definition for a given name.
+ * @param {?string} name Name of procedure to query.
+ * @return {?string} ID of procedure definition.
+ */
+Blocks.prototype.getProcedureDefinition = function (name) {
+    for (var id in this._blocks) {
+        var block = this._blocks[id];
+        if ((block.opcode == 'procedures_defnoreturn' ||
+            block.opcode == 'procedures_defreturn') &&
+            block.fields['NAME'].value == name) {
+            return id;
+        }
+    }
+    return null;
+};
+
 // ---------------------------------------------------------------------
 
 /**
@@ -226,12 +254,16 @@ Blocks.prototype.createBlock = function (block, opt_isFlyoutBlock) {
  */
 Blocks.prototype.changeBlock = function (args) {
     // Validate
-    if (args.element !== 'field') return;
+    if (args.element !== 'field' && args.element !== 'mutation') return;
     if (typeof this._blocks[args.id] === 'undefined') return;
-    if (typeof this._blocks[args.id].fields[args.name] === 'undefined') return;
 
-    // Update block value
-    this._blocks[args.id].fields[args.name].value = args.value;
+    if (args.element == 'field') {
+        // Update block value
+        if (!this._blocks[args.id].fields[args.name]) return;
+        this._blocks[args.id].fields[args.name].value = args.value;
+    } else if (args.element == 'mutation') {
+        this._blocks[args.id].mutation = mutationAdapter(args.value);
+    }
 };
 
 /**
@@ -355,6 +387,10 @@ Blocks.prototype.blockToXML = function (blockId) {
         ' type="' + block.opcode + '"' +
         xy +
         '>';
+    // Add any mutation. Must come before inputs.
+    if (block.mutation) {
+        xmlString += this.mutationToXML(block.mutation);
+    }
     // Add any inputs on this block.
     for (var input in block.inputs) {
         var blockInput = block.inputs[input];
@@ -389,6 +425,25 @@ Blocks.prototype.blockToXML = function (blockId) {
     return xmlString;
 };
 
+/**
+ * Recursively encode a mutation object to XML.
+ * @param {!Object} mutation Object representing a mutation.
+ * @return {string} XML string representing a mutation.
+ */
+Blocks.prototype.mutationToXML = function (mutation) {
+    var mutationString = '<' + mutation.tagName;
+    for (var prop in mutation) {
+        if (prop == 'children' || prop == 'tagName') continue;
+        mutationString += ' ' + prop + '="' + mutation[prop] + '"';
+    }
+    mutationString += '>';
+    for (var i = 0; i < mutation.children.length; i++) {
+        mutationString += this.mutationToXML(mutation.children[i]);
+    }
+    mutationString += '</' + mutation.tagName + '>';
+    return mutationString;
+};
+
 // ---------------------------------------------------------------------
 
 /**
diff --git a/src/engine/execute.js b/src/engine/execute.js
index 29116d5e5..59b4a3cbf 100644
--- a/src/engine/execute.js
+++ b/src/engine/execute.js
@@ -132,6 +132,12 @@ var execute = function (sequencer, thread) {
         argValues[inputName] = currentStackFrame.reported[inputName];
     }
 
+    // Add any mutation to args (e.g., for procedures).
+    var mutation = target.blocks.getMutation(currentBlockId);
+    if (mutation) {
+        argValues.mutation = mutation;
+    }
+
     // If we've gotten this far, all of the input blocks are evaluated,
     // and `argValues` is fully populated. So, execute the block primitive.
     // First, clear `currentStackFrame.reported`, so any subsequent execution
@@ -155,6 +161,9 @@ var execute = function (sequencer, thread) {
         startBranch: function (branchNum) {
             sequencer.stepToBranch(thread, branchNum);
         },
+        startProcedure: function (procedureName) {
+            sequencer.stepToProcedure(thread, procedureName);
+        },
         startHats: function(requestedHat, opt_matchFields, opt_target) {
             return (
                 runtime.startHats(requestedHat, opt_matchFields, opt_target)
diff --git a/src/engine/mutation-adapter.js b/src/engine/mutation-adapter.js
new file mode 100644
index 000000000..12dc123e1
--- /dev/null
+++ b/src/engine/mutation-adapter.js
@@ -0,0 +1,39 @@
+var html = require('htmlparser2');
+
+/**
+ * Adapter between mutator XML or DOM and block representation which can be
+ * used by the Scratch runtime.
+ * @param {(Object|string)} mutation Mutation XML string or DOM.
+ * @return {Object} Object representing the mutation.
+ */
+module.exports = function (mutation) {
+    var mutationParsed;
+    // Check if the mutation is already parsed; if not, parse it.
+    if (typeof mutation === 'object') {
+        mutationParsed = mutation;
+    } else {
+        mutationParsed = html.parseDOM(mutation)[0];
+    }
+    return mutatorTagToObject(mutationParsed);
+};
+
+/**
+ * Convert a part of a mutation DOM to a mutation VM object, recursively.
+ * @param {Object} dom DOM object for mutation tag.
+ * @return {Object} Object representing useful parts of this mutation.
+ */
+function mutatorTagToObject (dom) {
+    var obj = Object.create(null);
+    obj.tagName = dom.name;
+    obj.children = [];
+    for (var prop in dom.attribs) {
+        if (prop == 'xmlns') continue;
+        obj[prop] = dom.attribs[prop];
+    }
+    for (var i = 0; i < dom.children.length; i++) {
+        obj.children.push(
+            mutatorTagToObject(dom.children[i])
+        );
+    }
+    return obj;
+}
diff --git a/src/engine/runtime.js b/src/engine/runtime.js
index 75067be3d..18cffd8ec 100644
--- a/src/engine/runtime.js
+++ b/src/engine/runtime.js
@@ -15,7 +15,8 @@ var defaultBlockPackages = {
     'scratch3_motion': require('../blocks/scratch3_motion'),
     'scratch3_operators': require('../blocks/scratch3_operators'),
     'scratch3_sensing': require('../blocks/scratch3_sensing'),
-    'scratch3_data': require('../blocks/scratch3_data')
+    'scratch3_data': require('../blocks/scratch3_data'),
+    'scratch3_procedures': require('../blocks/scratch3_procedures')
 };
 
 /**
diff --git a/src/engine/sequencer.js b/src/engine/sequencer.js
index 690ab61a7..85575c804 100644
--- a/src/engine/sequencer.js
+++ b/src/engine/sequencer.js
@@ -124,6 +124,16 @@ Sequencer.prototype.stepToBranch = function (thread, branchNum) {
     }
 };
 
+/**
+ * Step a procedure.
+ * @param {!Thread} thread Thread object to step to procedure.
+ * @param {!string} procedureName Name of procedure defined in this target.
+ */
+Sequencer.prototype.stepToProcedure = function (thread, procedureName) {
+    var definition = thread.target.blocks.getProcedureDefinition(procedureName);
+    thread.pushStack(definition);
+};
+
 /**
  * Step a thread into an input reporter, and manage its status appropriately.
  * @param {!Thread} thread Thread object to step to reporter.
diff --git a/src/import/sb2import.js b/src/import/sb2import.js
index 79e135a7b..f633de9c0 100644
--- a/src/import/sb2import.js
+++ b/src/import/sb2import.js
@@ -327,6 +327,14 @@ function parseBlock (sb2block) {
             };
         }
     }
+    // Special cases to generate mutations.
+    if (oldOpcode == 'call') {
+        activeBlock.mutation = {
+            tagName: 'mutation',
+            children: [],
+            name: sb2block[1]
+        };
+    }
     return activeBlock;
 }
 
diff --git a/src/import/sb2specmap.js b/src/import/sb2specmap.js
index 693d82f81..76e9683e1 100644
--- a/src/import/sb2specmap.js
+++ b/src/import/sb2specmap.js
@@ -1374,15 +1374,20 @@ var specMap = {
         ]
     },
     'procDef':{
-        'opcode':'proc_def',
-        'argMap':[]
+        'opcode':'procedures_defnoreturn',
+        'argMap':[
+            {
+                'type':'field',
+                'fieldName':'NAME'
+            }
+        ]
     },
     'getParam':{
         'opcode':'proc_param',
         'argMap':[]
     },
     'call':{
-        'opcode':'proc_call',
+        'opcode':'procedures_callnoreturn',
         'argMap':[]
     }
 };