diff --git a/.eslintrc b/.eslintrc
index 96821cc6..74c9e4d3 100644
--- a/.eslintrc
+++ b/.eslintrc
@@ -39,7 +39,7 @@
         "no-with": 2,
         "one-var": [2, "never"],
         "prefer-const": 2,
-        "prefer-spread": 2,
+        "prefer-spread": 0, // re-enable once we use es6
         "semi": [2, "always"],
         "space-before-blocks": 2,
         "space-before-function-paren": [2, "never"],
diff --git a/package.json b/package.json
index 4f00ff38..6e794ffe 100644
--- a/package.json
+++ b/package.json
@@ -30,6 +30,7 @@
     "less": "~2.7.1",
     "morgan": "^1.7.0",
     "nomnom": "^1.8.1",
+    "object-assign": "^4.1.0",
     "pako": "1.0.4",
     "selenium-webdriver": "^2.48.2",
     "sri-toolbox": "^0.2.0",
diff --git a/src/MacroExpander.js b/src/MacroExpander.js
index 8569e6ad..608feafe 100644
--- a/src/MacroExpander.js
+++ b/src/MacroExpander.js
@@ -4,16 +4,23 @@
  */
 
 const Lexer = require("./Lexer");
+const builtinMacros = require("./macros");
+const ParseError = require("./ParseError");
+const objectAssign = require("object-assign");
 
 function MacroExpander(input, macros) {
     this.lexer = new Lexer(input);
-    this.macros = macros;
+    this.macros = objectAssign({}, builtinMacros, macros);
     this.stack = []; // contains tokens in REVERSE order
     this.discardedWhiteSpace = [];
 }
 
 /**
  * Recursively expand first token, then return first non-expandable token.
+ *
+ * At the moment, macro expansion doesn't handle delimited macros,
+ * i.e. things like those defined by \def\foo#1\end{…}.
+ * See the TeX book page 202ff. for details on how those should behave.
  */
 MacroExpander.prototype.nextToken = function() {
     for (;;) {
@@ -25,18 +32,87 @@ MacroExpander.prototype.nextToken = function() {
         if (!(name.charAt(0) === "\\" && this.macros.hasOwnProperty(name))) {
             return topToken;
         }
+        let tok;
         let expansion = this.macros[name];
         if (typeof expansion === "string") {
+            let numArgs = 0;
+            if (expansion.indexOf("#") !== -1) {
+                const stripped = expansion.replace(/##/g, "");
+                while (stripped.indexOf("#" + (numArgs + 1)) !== -1) {
+                    ++numArgs;
+                }
+            }
             const bodyLexer = new Lexer(expansion);
             expansion = [];
-            let tok = bodyLexer.lex();
+            tok = bodyLexer.lex();
             while (tok.text !== "EOF") {
                 expansion.push(tok);
                 tok = bodyLexer.lex();
             }
             expansion.reverse(); // to fit in with stack using push and pop
+            expansion.numArgs = numArgs;
             this.macros[name] = expansion;
         }
+        if (expansion.numArgs) {
+            const args = [];
+            let i;
+            // obtain arguments, either single token or balanced {…} group
+            for (i = 0; i < expansion.numArgs; ++i) {
+                const startOfArg = this.get(true);
+                if (startOfArg.text === "{") {
+                    const arg = [];
+                    let depth = 1;
+                    while (depth !== 0) {
+                        tok = this.get(false);
+                        arg.push(tok);
+                        if (tok.text === "{") {
+                            ++depth;
+                        } else if (tok.text === "}") {
+                            --depth;
+                        } else if (tok.text === "EOF") {
+                            throw new ParseError(
+                                "End of input in macro argument",
+                                startOfArg);
+                        }
+                    }
+                    arg.pop(); // remove last }
+                    arg.reverse(); // like above, to fit in with stack order
+                    args[i] = arg;
+                } else if (startOfArg.text === "EOF") {
+                    throw new ParseError(
+                        "End of input expecting macro argument", topToken);
+                } else {
+                    args[i] = [startOfArg];
+                }
+            }
+            // paste arguments in place of the placeholders
+            expansion = expansion.slice(); // make a shallow copy
+            for (i = expansion.length - 1; i >= 0; --i) {
+                tok = expansion[i];
+                if (tok.text === "#") {
+                    if (i === 0) {
+                        throw new ParseError(
+                            "Incomplete placeholder at end of macro body",
+                            tok);
+                    }
+                    tok = expansion[--i]; // next token on stack
+                    if (tok.text === "#") { // ## → #
+                        expansion.splice(i + 1, 1); // drop first #
+                    } else if (/^[1-9]$/.test(tok.text)) {
+                        // expansion.splice(i, 2, arg[0], arg[1], …)
+                        // to replace placeholder with the indicated argument.
+                        // TODO: use spread once we move to ES2015
+                        expansion.splice.apply(
+                            expansion,
+                            [i, 2].concat(args[tok.text - 1]));
+                    } else {
+                        throw new ParseError(
+                            "Not a valid argument number",
+                            tok);
+                    }
+                }
+            }
+        }
         this.stack = this.stack.concat(expansion);
     }
 };
diff --git a/src/macros.js b/src/macros.js
new file mode 100644
index 00000000..fb1054bf
--- /dev/null
+++ b/src/macros.js
@@ -0,0 +1,23 @@
+/**
+ * Predefined macros for KaTeX.
+ * This can be used to define some commands in terms of others.
+ */
+
+// This function might one day accept additional argument and do more things.
+function defineMacro(name, body) {
+    module.exports[name] = body;
+}
+
+//////////////////////////////////////////////////////////////////////
+// basics
+defineMacro("\\bgroup", "{");
+defineMacro("\\egroup", "}");
+defineMacro("\\begingroup", "{");
+defineMacro("\\endgroup", "}");
+
+//////////////////////////////////////////////////////////////////////
+// amsmath.sty
+
+// \def\overset#1#2{\binrel@{#2}\binrel@@{\mathop{\kern\z@#2}\limits^{#1}}}
+defineMacro("\\overset", "\\mathop{#2}\\limits^{#1}");
+defineMacro("\\underset", "\\mathop{#2}\\limits_{#1}");
diff --git a/test/katex-spec.js b/test/katex-spec.js
index 36dd5f5b..651df70e 100644
--- a/test/katex-spec.js
+++ b/test/katex-spec.js
@@ -2011,6 +2011,13 @@ describe("A macro expander", function() {
             "\\bar": "a",
         });
     });
+
+    it("should expand the \overset macro as expected", function() {
+        expect("\\overset?=").toParseLike("\\mathop{=}\\limits^{?}");
+        expect("\\overset{x=y}{\sqrt{ab}}")
+            .toParseLike("\\mathop{\sqrt{ab}}\\limits^{x=y}");
+        expect("\\overset {?} =").toParseLike("\\mathop{=}\\limits^{?}");
+    });
 });
 
 describe("A parser taking String objects", function() {
diff --git a/test/screenshotter/images/GroupMacros-chrome.png b/test/screenshotter/images/GroupMacros-chrome.png
new file mode 100644
index 00000000..da77a3ac
Binary files /dev/null and b/test/screenshotter/images/GroupMacros-chrome.png differ
diff --git a/test/screenshotter/images/GroupMacros-firefox.png b/test/screenshotter/images/GroupMacros-firefox.png
new file mode 100644
index 00000000..1813705b
Binary files /dev/null and b/test/screenshotter/images/GroupMacros-firefox.png differ
diff --git a/test/screenshotter/images/OverUnderset-chrome.png b/test/screenshotter/images/OverUnderset-chrome.png
new file mode 100644
index 00000000..3c5c71d7
Binary files /dev/null and b/test/screenshotter/images/OverUnderset-chrome.png differ
diff --git a/test/screenshotter/images/OverUnderset-firefox.png b/test/screenshotter/images/OverUnderset-firefox.png
new file mode 100644
index 00000000..6ffe2e20
Binary files /dev/null and b/test/screenshotter/images/OverUnderset-firefox.png differ
diff --git a/test/screenshotter/ss_data.js b/test/screenshotter/ss_data.js
index 7dd926da..81ff909f 100644
--- a/test/screenshotter/ss_data.js
+++ b/test/screenshotter/ss_data.js
@@ -26,5 +26,8 @@ for (var key in dict) {
         }
     });
     itm.query = querystring.stringify(query);
+    if (itm.macros) {
+        itm.query += "&" + querystring.stringify(itm.macros);
+    }
 }
 module.exports = dict;
diff --git a/test/screenshotter/ss_data.yaml b/test/screenshotter/ss_data.yaml
index db415846..51e6373d 100644
--- a/test/screenshotter/ss_data.yaml
+++ b/test/screenshotter/ss_data.yaml
@@ -74,6 +74,11 @@ Exponents: a^{a^a_a}_{a^a_a}
 FractionTest: \dfrac{a}{b}\frac{a}{b}\tfrac{a}{b}\;-\dfrac12\;1\tfrac12\;{1 \atop 2}
 Functions: \sin\cos\tan\ln\log
 GreekLetters: \alpha\beta\gamma\omega
+GroupMacros:
+    macros:
+        \startExp: e^\bgroup
+        \endExp: \egroup
+    tex: \startExp a+b\endExp
 KaTeX: \KaTeX
 Kern:
     tex: \frac{a\kern{1em}b}{c}a\kern{1em}b\kern{1ex}c\kern{-0.25em}d
@@ -128,6 +133,13 @@ OpLimits: |
     {\sin_2^2 \lim_2^2 \int_2^2 \sum_2^2}
     {\displaystyle \lim_2^2 \int_2^2 \intop_2^2 \sum_2^2}
 OverUnderline: x\underline{x}\underline{\underline{x}}\underline{x_{x_{x_x}}}\underline{x^{x^{x^x}}}\overline{x}\overline{x}\overline{x^{x^{x^x}}} \blue{\overline{\underline{x}}\underline{\overline{x}}}
+OverUnderset: |
+    \begin{array}{l}
+    x\overset?=1\\
+    {\displaystyle\lim_{t\underset{>0}\to0}}\\
+    a+b+c+d\overset{b+c=0}\longrightarrow a+d\\
+    \overset { x = y } { \sqrt { a b } }
+    \end{array}
 Phantom: \dfrac{1+\phantom{x^{\blue{2}}} = x}{1+x^{\blue{2}} = x}
 PrimeSpacing: f'+f_2'+f^{f'}
 PrimeSuper: x'^2+x'''^2+x'^2_3+x_3'^2
diff --git a/test/screenshotter/test.html b/test/screenshotter/test.html
index 4d9745fe..b046e42a 100644
--- a/test/screenshotter/test.html
+++ b/test/screenshotter/test.html
@@ -47,6 +47,12 @@
       if (query["errorColor"]) {
           settings.errorColor = query["errorColor"];
       }
+      var macros = {};
+      var macroRegex = /(?:^\?|&)(?:\\|%5[Cc])([A-Za-z]+)=([^&]*)/g;
+      while ((match = macroRegex.exec(window.location.search)) !== null) {
+          settings.macros = macros;
+          macros["\\" + match[1]] = decodeURIComponent(match[2]);
+      }
 
       katex.render(query["tex"], mathNode, settings);
       document.getElementById("pre").innerHTML = query["pre"] || "";