diff --git a/api/controller.js b/api/controller.js
index 949731c..adf878a 100644
--- a/api/controller.js
+++ b/api/controller.js
@@ -16,6 +16,7 @@ exports.calculate = function(req, res) {
     'subtract': function(a, b) { return a - b },
     'multiply': function(a, b) { return a * b },
     'divide':   function(a, b) { return a / b },
+    'modulo':   function(a, b) { return a % b },
   };
 
   if (!req.query.operation) {
diff --git a/test/arithmetic.test.js b/test/arithmetic.test.js
index deded48..b4eca1a 100644
--- a/test/arithmetic.test.js
+++ b/test/arithmetic.test.js
@@ -95,6 +95,16 @@ describe('Arithmetic', function () {
 
 // TODO: Challenge #1
  
+    describe('Subtraction', function () {
+        it('subtracts a positive integer from a positive integer', function (done) {
+            request.get('/arithmetic?operation=subtract&operand1=42&operand2=21')
+                .expect(200)
+                .end(function (err, res) {
+                    expect(res.body).to.eql({ result: 21 });
+                    done();
+                });
+        });
+    });
 
     describe('Multiplication', function () {
         it('multiplies two positive integers', function (done) {
@@ -205,4 +215,47 @@ describe('Arithmetic', function () {
                 });
         });
     });
+
+    describe('Modulo', function () {
+        it('computes modulo of two positive integers', function (done) {
+            request.get('/arithmetic?operation=modulo&operand1=42&operand2=5')
+                .expect(200)
+                .end(function (err, res) {
+                    expect(res.body).to.eql({ result: 2 });
+                    done();
+                });
+        });
+        it('computes modulo with a result of zero', function (done) {
+            request.get('/arithmetic?operation=modulo&operand1=42&operand2=21')
+                .expect(200)
+                .end(function (err, res) {
+                    expect(res.body).to.eql({ result: 0 });
+                    done();
+                });
+        });
+        it('computes modulo of a negative integer', function (done) {
+            request.get('/arithmetic?operation=modulo&operand1=-42&operand2=5')
+                .expect(200)
+                .end(function (err, res) {
+                    expect(res.body).to.eql({ result: -2 });
+                    done();
+                });
+        });
+        it('computes modulo with floating point numbers', function (done) {
+            request.get('/arithmetic?operation=modulo&operand1=5.5&operand2=2')
+                .expect(200)
+                .end(function (err, res) {
+                    expect(res.body).to.eql({ result: 1.5 });
+                    done();
+                });
+        });
+        it('computes modulo where divisor is larger than dividend', function (done) {
+            request.get('/arithmetic?operation=modulo&operand1=5&operand2=42')
+                .expect(200)
+                .end(function (err, res) {
+                    expect(res.body).to.eql({ result: 5 });
+                    done();
+                });
+        });
+    });
 });