diff --git a/src/function/LispFunction.java b/src/function/LispFunction.java
index 4372f3e..6b6704e 100644
--- a/src/function/LispFunction.java
+++ b/src/function/LispFunction.java
@@ -5,7 +5,7 @@ import sexpression.SExpression;
 
 public abstract class LispFunction {
 
-    public abstract SExpression call(Cons argList);
+    public abstract SExpression call(Cons argumentList);
 
     public boolean isArgumentListEvaluated() {
         return true;
diff --git a/src/function/builtin/math/MODULO.java b/src/function/builtin/math/MODULO.java
new file mode 100644
index 0000000..b092b96
--- /dev/null
+++ b/src/function/builtin/math/MODULO.java
@@ -0,0 +1,45 @@
+package function.builtin.math;
+
+import error.LispException;
+import function.ArgumentValidator;
+import function.FunctionNames;
+import function.LispFunction;
+import sexpression.Cons;
+import sexpression.LispNumber;
+import sexpression.SExpression;
+
+@FunctionNames({ "MOD", "MODULO", "%" })
+public class MODULO extends LispFunction {
+
+    private ArgumentValidator argumentValidator;
+
+    public MODULO(String name) {
+        this.argumentValidator = new ArgumentValidator(name);
+        this.argumentValidator.setExactNumberOfArguments(2);
+        this.argumentValidator.setEveryArgumentExpectedType(LispNumber.class);
+    }
+
+    @Override
+    public SExpression call(Cons argumentList) {
+        argumentValidator.validate(argumentList);
+        LispNumber dividend = (LispNumber) argumentList.getFirst();
+        LispNumber divisor = (LispNumber) ((Cons) argumentList.getRest()).getFirst();
+
+        try {
+            return new LispNumber(dividend.getValue().mod(divisor.getValue()));
+        } catch (ArithmeticException e) {
+            throw new ModulusNotPositiveException();
+        }
+    }
+
+    public static class ModulusNotPositiveException extends LispException {
+
+        private static final long serialVersionUID = 1L;
+
+        @Override
+        public String getMessage() {
+            return "modulus not positive";
+        }
+    }
+
+}
diff --git a/src/table/FunctionTable.java b/src/table/FunctionTable.java
index ac69719..57932e1 100644
--- a/src/table/FunctionTable.java
+++ b/src/table/FunctionTable.java
@@ -26,6 +26,7 @@ import function.builtin.cons.LIST;
 import function.builtin.cons.REST;
 import function.builtin.math.DIVIDE;
 import function.builtin.math.MINUS;
+import function.builtin.math.MODULO;
 import function.builtin.math.MULTIPLY;
 import function.builtin.math.PLUS;
 import function.builtin.predicate.ATOM;
@@ -88,6 +89,7 @@ public class FunctionTable {
         allBuiltIns.add(LISTP.class);
         allBuiltIns.add(LOAD.class);
         allBuiltIns.add(MINUS.class);
+        allBuiltIns.add(MODULO.class);
         allBuiltIns.add(MULTIPLY.class);
         allBuiltIns.add(NULL.class);
         allBuiltIns.add(OR.class);
diff --git a/test/function/LispFunctionTest.java b/test/function/LispFunctionTest.java
index 20243fc..7bb4760 100644
--- a/test/function/LispFunctionTest.java
+++ b/test/function/LispFunctionTest.java
@@ -14,7 +14,7 @@ public class LispFunctionTest {
         LispFunction lispFunction = new LispFunction() {
 
             @Override
-            public SExpression call(Cons argList) {
+            public SExpression call(Cons argumentList) {
                 return null;
             }
         };
diff --git a/test/function/LispSpecialFunctionTest.java b/test/function/LispSpecialFunctionTest.java
index 9ff4622..87c1b6a 100644
--- a/test/function/LispSpecialFunctionTest.java
+++ b/test/function/LispSpecialFunctionTest.java
@@ -14,7 +14,7 @@ public class LispSpecialFunctionTest {
         LispFunction lispFunction = new LispSpecialFunction() {
 
             @Override
-            public SExpression call(Cons argList) {
+            public SExpression call(Cons argumentList) {
                 return null;
             }
         };
diff --git a/test/function/builtin/math/MODULOTest.java b/test/function/builtin/math/MODULOTest.java
new file mode 100644
index 0000000..efe838b
--- /dev/null
+++ b/test/function/builtin/math/MODULOTest.java
@@ -0,0 +1,115 @@
+package function.builtin.math;
+
+import static testutil.TestUtilities.assertSExpressionsMatch;
+import static testutil.TestUtilities.evaluateString;
+
+import org.junit.Test;
+
+import function.ArgumentValidator.BadArgumentTypeException;
+import function.ArgumentValidator.TooFewArgumentsException;
+import function.ArgumentValidator.TooManyArgumentsException;
+import function.builtin.math.MODULO.ModulusNotPositiveException;
+import sexpression.LispNumber;
+import testutil.SymbolAndFunctionCleaner;
+
+public class MODULOTest extends SymbolAndFunctionCleaner {
+
+    @Test
+    public void mod() {
+        String input = "(mod 5 3)";
+
+        assertSExpressionsMatch(new LispNumber("2"), evaluateString(input));
+    }
+
+    @Test
+    public void modulo() {
+        String input = "(modulo 11 7)";
+
+        assertSExpressionsMatch(new LispNumber("4"), evaluateString(input));
+    }
+
+    @Test
+    public void moduloSymbol() {
+        String input = "(% 8 5)";
+
+        assertSExpressionsMatch(new LispNumber("3"), evaluateString(input));
+    }
+
+    @Test
+    public void dividendGreaterThanDivisor() {
+        String input = "(mod 21 19)";
+
+        assertSExpressionsMatch(new LispNumber("2"), evaluateString(input));
+    }
+
+    @Test
+    public void dividendLessThanDivisor() {
+        String input = "(mod 5 239)";
+
+        assertSExpressionsMatch(new LispNumber("5"), evaluateString(input));
+    }
+
+    @Test
+    public void dividendEqualToDivisor() {
+        String input = "(mod 5 5)";
+
+        assertSExpressionsMatch(new LispNumber("0"), evaluateString(input));
+    }
+
+    @Test
+    public void dividendMultipleOfDivisor() {
+        String input = "(mod 20 5)";
+
+        assertSExpressionsMatch(new LispNumber("0"), evaluateString(input));
+    }
+
+    @Test
+    public void divisorOfOne() {
+        String input = "(mod 5 1)";
+
+        assertSExpressionsMatch(new LispNumber("0"), evaluateString(input));
+    }
+
+    @Test
+    public void dividendOfZero() {
+        String input = "(mod 0 2309)";
+
+        assertSExpressionsMatch(new LispNumber("0"), evaluateString(input));
+    }
+
+    @Test
+    public void negativeDividend() {
+        String input = "(mod -23 25)";
+
+        assertSExpressionsMatch(new LispNumber("2"), evaluateString(input));
+    }
+
+    @Test(expected = ModulusNotPositiveException.class)
+    public void divisorOfZero() {
+        String input = "(mod 5 0)";
+
+        assertSExpressionsMatch(new LispNumber("0"), evaluateString(input));
+    }
+
+    @Test(expected = ModulusNotPositiveException.class)
+    public void negativeDivisor() {
+        String input = "(mod 5 -10)";
+
+        assertSExpressionsMatch(new LispNumber("0"), evaluateString(input));
+    }
+
+    @Test(expected = BadArgumentTypeException.class)
+    public void modWithNonNumber() {
+        evaluateString("(mod 'a 'b)");
+    }
+
+    @Test(expected = TooFewArgumentsException.class)
+    public void modWithTooFewArguments() {
+        evaluateString("(mod 1)");
+    }
+
+    @Test(expected = TooManyArgumentsException.class)
+    public void modWithTooManyArguments() {
+        evaluateString("(mod 1 2 3)");
+    }
+}
diff --git a/test/table/FunctionTableTest.java b/test/table/FunctionTableTest.java
index 0e24546..254d023 100644
--- a/test/table/FunctionTableTest.java
+++ b/test/table/FunctionTableTest.java
@@ -34,7 +34,7 @@ public class FunctionTableTest {
         public GoodFunction(String name) {}
 
         @Override
-        public SExpression call(Cons argList) {
+        public SExpression call(Cons argumentList) {
             return NIL;
         }
     }
@@ -43,7 +43,7 @@ public class FunctionTableTest {
     public static class BadFunction extends LispFunction {
 
         @Override
-        public SExpression call(Cons argList) {
+        public SExpression call(Cons argumentList) {
             return NIL;
         }
     }
@@ -51,7 +51,7 @@ public class FunctionTableTest {
     public static class UglyFunction extends LispFunction {
 
         @Override
-        public SExpression call(Cons argList) {
+        public SExpression call(Cons argumentList) {
             return NIL;
         }
     }
@@ -60,7 +60,7 @@ public class FunctionTableTest {
         return new LispFunction() {
 
             @Override
-            public SExpression call(Cons argList) {
+            public SExpression call(Cons argumentList) {
                 return T;
             }
         };