diff --git a/README.md b/README.md
index d2215a7..db66371 100644
--- a/README.md
+++ b/README.md
@@ -15,6 +15,7 @@ structures:
 * [Category](#category)
 * [Semigroup](#semigroup)
 * [Monoid](#monoid)
+* [Group](#group)
 * [Functor](#functor)
 * [Contravariant](#contravariant)
 * [Apply](#apply)
@@ -306,6 +307,26 @@ Given a value `m`, one can access its type representative via the
 
 1. `empty` must return a value of the same Monoid
 
+### Group
+
+A value that implements the Group specification must also implement
+the [Monoid](#monoid) specification.
+
+1. `g.concat(g.invert())` is equivalent to `g.empty()` (right inverse)
+2. `g.invert().concat(g)` is equivalent to `g.empty()` (left inverse)
+
+#### `invert` method
+
+```hs
+invert :: Group g => g ~> () -> g
+```
+A value which has a Group must provide an `invert` method. The
+`invert` method takes no arguments:
+
+    g.invert()
+
+1. `invert` must return a value of the same Group.
+
 ### Functor
 
 1. `u.map(a => a)` is equivalent to `u` (identity)
diff --git a/figures/dependencies.dot b/figures/dependencies.dot
index cc5122b..d632f3f 100644
--- a/figures/dependencies.dot
+++ b/figures/dependencies.dot
@@ -14,6 +14,7 @@ digraph {
   Extend;
   Foldable;
   Functor;
+  Group;
   Contravariant;
   Monad;
   Monoid;
@@ -41,6 +42,7 @@ digraph {
   Functor -> Extend;
   Functor -> Profunctor;
   Functor -> Traversable;
+  Monoid -> Group;
   Plus -> Alternative;
   Semigroup -> Monoid;
   Semigroupoid -> Category;
diff --git a/figures/dependencies.png b/figures/dependencies.png
index 598c70a..e4d814b 100644
Binary files a/figures/dependencies.png and b/figures/dependencies.png differ
diff --git a/index.js b/index.js
index 14415f3..6e0e13e 100644
--- a/index.js
+++ b/index.js
@@ -12,6 +12,7 @@
     id: 'fantasy-land/id',
     concat: 'fantasy-land/concat',
     empty: 'fantasy-land/empty',
+    invert: 'fantasy-land/invert',
     map: 'fantasy-land/map',
     contramap: 'fantasy-land/contramap',
     ap: 'fantasy-land/ap',
diff --git a/internal/index.js b/internal/index.js
index dc8f356..b433c21 100644
--- a/internal/index.js
+++ b/internal/index.js
@@ -2,7 +2,7 @@
 
 const patch = require('./patch');
 const Id = require('./id');
-const Sum = require('./string_sum');
+const Sum = require('./sum');
 const Compose = require('./compose');
 const {equality} = require('./func');
 
diff --git a/internal/string_sum.js b/internal/sum.js
similarity index 73%
rename from internal/string_sum.js
rename to internal/sum.js
index 9285b35..83053dc 100644
--- a/internal/string_sum.js
+++ b/internal/sum.js
@@ -5,10 +5,10 @@ const {tagged} = require('daggy');
 const fl = require('..');
 const {equality} = require('./func');
 
-// Special type of sum for the type of string.
 const Sum = module.exports = tagged('v');
+
 Sum[fl.of] = Sum;
-Sum[fl.empty] = () => Sum('');
+Sum[fl.empty] = () => Sum(0);
 Sum.prototype[fl.map] = function(f) {
   return Sum(f(this.v));
 };
@@ -18,3 +18,6 @@ Sum.prototype[fl.concat] = function(x) {
 Sum.prototype[fl.equals] = function(x) {
   return equality(this.v, x.v);
 };
+Sum.prototype[fl.invert] = function() {
+  return Sum(this.v >= 0 ? -Math.abs(this.v) : Math.abs(this.v));
+};
diff --git a/laws/group.js b/laws/group.js
new file mode 100644
index 0000000..9410157
--- /dev/null
+++ b/laws/group.js
@@ -0,0 +1,30 @@
+'use strict';
+
+const {of, empty, concat, invert} = require('..');
+
+/**
+
+### Group
+
+1. `g.concat(g.invert())` is equivalent to `g.empty()` (right inverse)
+2. `g.invert().concat(g)` is equivalent to `g.empty()` (left inverse)
+
+**/
+
+const rightInverse = T => eq => x => {
+  const g = T[of](x);
+
+  const a = g[concat](g[invert]());
+  const b = T[empty]();
+  return eq(a, b);
+};
+
+const leftInverse = T => eq => x => {
+  const g = T[of](x);
+
+  const a = g[invert]()[concat](g);
+  const b = T[empty]();
+  return eq(a, b);
+};
+
+module.exports = {rightInverse, leftInverse};
diff --git a/test.js b/test.js
index 0b3fb8a..93d5fd0 100644
--- a/test.js
+++ b/test.js
@@ -14,6 +14,7 @@ const comonad = require('./laws/comonad');
 const extend = require('./laws/extend');
 const foldable = require('./laws/foldable');
 const functor = require('./laws/functor');
+const group = require('./laws/group');
 const monad = require('./laws/monad');
 const monoid = require('./laws/monoid');
 const ord = require('./laws/ord');
@@ -90,6 +91,11 @@ exports.functor = {
   composition: test(functor.composition(Id[fl.of])(equality)(a => [a, a])(a => [a])),
 };
 
+exports.group = {
+  rightInverse: test(() => group.rightInverse(Sum[fl.of])(equality)(42)),
+  leftInverse: test(() => group.leftInverse(Sum[fl.of])(equality)(42)),
+};
+
 exports.monad = {
   leftIdentity: test(monad.leftIdentity(Id)(equality)(Id[fl.of])),
   rightIdentity: test(monad.rightIdentity(Id)(equality)),
@@ -102,8 +108,8 @@ exports.plus = {
 };
 
 exports.monoid = {
-  leftIdentity: test(monoid.leftIdentity(Sum)(equality)),
-  rightIdentity: test(monoid.rightIdentity(Sum)(equality)),
+  leftIdentity: test(() => monoid.leftIdentity(Sum)(equality)(23)),
+  rightIdentity: test(() => monoid.rightIdentity(Sum)(equality)(23)),
 };
 
 exports.ord = {
@@ -117,7 +123,7 @@ exports.semigroup = {
 };
 
 exports.semigroupoid = {
-  associativity: semigroupoid.associativity(x => x + 1)(x => x * x)(x => x - 2)(equality)(5),
+  associativity: test(() => semigroupoid.associativity(x => x + 1)(x => x * x)(x => x - 2)(equality)(5)),
 };
 
 exports.setoid = {
@@ -129,5 +135,5 @@ exports.setoid = {
 exports.traversable = {
   naturality: test(x => traversable.naturality(Id)(Id[fl.of])(equality)(Id[fl.of](x))),
   identity: test(traversable.identity(Id)(equality)),
-  composition: test(x => traversable.composition(Id)(Id[fl.of])(equality)(Id[fl.of](Sum[fl.of](x)))),
+  composition: test(() => traversable.composition(Id)(Id[fl.of])(equality)(Id[fl.of](Sum[fl.of](37)))),
 };