diff --git a/src/function/builtin/DIVIDE.java b/src/function/builtin/DIVIDE.java
index aaf8b57..c8bd044 100644
--- a/src/function/builtin/DIVIDE.java
+++ b/src/function/builtin/DIVIDE.java
@@ -1,5 +1,7 @@
 package function.builtin;
 
+import java.math.BigInteger;
+
 import function.LispFunction;
 import sexpression.*;
 
@@ -26,7 +28,7 @@ public class DIVIDE extends LispFunction {
             if (argRest.nullp()) {
                 // there is only one argument, so return the multiplicative
                 // inverse of the number
-                return new LispNumber(1 / num1.getValue());
+                return new LispNumber(BigInteger.ONE.divide(num1.getValue()));
             }
 
             SExpression argSecond = argRest.getCar();
@@ -34,7 +36,7 @@ public class DIVIDE extends LispFunction {
             // make sure that the next argument is a number as well
             if (argSecond.numberp()) {
                 LispNumber num2 = (LispNumber) argSecond;
-                LispNumber quotient = new LispNumber(num1.getValue() / num2.getValue());
+                LispNumber quotient = new LispNumber(num1.getValue().divide(num2.getValue()));
                 SExpression argCddr = argRest.getCdr();
 
                 if (argCddr.consp()) {
diff --git a/src/function/builtin/EQUALSP.java b/src/function/builtin/EQUALSP.java
index 6bdb456..deff065 100644
--- a/src/function/builtin/EQUALSP.java
+++ b/src/function/builtin/EQUALSP.java
@@ -33,7 +33,7 @@ public class EQUALSP extends LispFunction {
             if (secondArg.numberp()) {
                 LispNumber num2 = (LispNumber) secondArg;
 
-                if (num1.getValue() == num2.getValue()) {
+                if (num1.getValue().equals(num2.getValue())) {
                     return call(argRest);
                 }
 
diff --git a/src/function/builtin/GREATERP.java b/src/function/builtin/GREATERP.java
index 955f403..fed1bbf 100644
--- a/src/function/builtin/GREATERP.java
+++ b/src/function/builtin/GREATERP.java
@@ -33,7 +33,7 @@ public class GREATERP extends LispFunction {
             if (secondArg.numberp()) {
                 LispNumber num2 = (LispNumber) secondArg;
 
-                if (num1.getValue() > num2.getValue()) {
+                if (num1.getValue().compareTo(num2.getValue()) > 0) {
                     return call(argRest);
                 }
 
diff --git a/src/function/builtin/LENGTH.java b/src/function/builtin/LENGTH.java
index 353e312..b4b2d64 100644
--- a/src/function/builtin/LENGTH.java
+++ b/src/function/builtin/LENGTH.java
@@ -1,5 +1,7 @@
 package function.builtin;
 
+import java.math.BigInteger;
+
 import function.LispFunction;
 import sexpression.*;
 
@@ -12,15 +14,15 @@ public class LENGTH extends LispFunction {
      * Returns the length of the given list.
      *
      * @param list
-     *  the list to determine the length of
-     * @return
-     *  the length of <code>list</code>
+     *            the list to determine the length of
+     * @return the length of <code>list</code>
      */
     public static int getLength(Cons list) {
         LENGTH lengthFunction = new LENGTH();
         LispNumber length = lengthFunction.call(LIST.makeList(list));
 
-        return length.getValue();
+        return length.getValue().intValue(); // TODO - return BigInteger when all built-ins use
+                                             // ArgumentValidator
     }
 
     public LispNumber call(Cons argList) {
@@ -28,19 +30,17 @@ public class LENGTH extends LispFunction {
         if (argList.nullp()) {
             Cons originalSExpr = new Cons(new Symbol("LENGTH"), argList);
 
-            throw new RuntimeException("too few arguments given to LENGTH: " +
-                                       originalSExpr);
+            throw new RuntimeException("too few arguments given to LENGTH: " + originalSExpr);
         }
 
         SExpression argCar = argList.getCar();
         SExpression argCdr = argList.getCdr();
 
         // make sure we have received only one argument
-        if (! argCdr.nullp()) {
+        if (!argCdr.nullp()) {
             Cons originalSExpr = new Cons(new Symbol("LENGTH"), argList);
 
-            throw new RuntimeException("too many arguments given to LENGTH: " +
-                                       originalSExpr);
+            throw new RuntimeException("too many arguments given to LENGTH: " + originalSExpr);
         }
 
         // make sure that the argument is a list
@@ -48,17 +48,16 @@ public class LENGTH extends LispFunction {
             Cons arg = (Cons) argCar;
 
             if (arg.nullp()) {
-                return new LispNumber(0);
+                return new LispNumber(BigInteger.ZERO);
             }
 
             Cons cdr = LIST.makeList(arg.getCdr());
             LispNumber cdrLength = call(cdr);
 
-            return new LispNumber(1 + cdrLength.getValue());
+            return new LispNumber(BigInteger.ONE.add(cdrLength.getValue()));
         }
 
-        throw new RuntimeException("LENGTH: a proper list must not end with " +
-                                   argCar);
+        throw new RuntimeException("LENGTH: a proper list must not end with " + argCar);
     }
 
 }
diff --git a/src/function/builtin/LESSP.java b/src/function/builtin/LESSP.java
index 8746c31..9275327 100644
--- a/src/function/builtin/LESSP.java
+++ b/src/function/builtin/LESSP.java
@@ -33,7 +33,7 @@ public class LESSP extends LispFunction {
             if (secondArg.numberp()) {
                 LispNumber num2 = (LispNumber) secondArg;
 
-                if (num1.getValue() < num2.getValue()) {
+                if (num1.getValue().compareTo(num2.getValue()) < 0) {
                     return call(argRest);
                 }
 
diff --git a/src/function/builtin/MINUS.java b/src/function/builtin/MINUS.java
index cbe15dc..d9b4165 100644
--- a/src/function/builtin/MINUS.java
+++ b/src/function/builtin/MINUS.java
@@ -1,5 +1,7 @@
 package function.builtin;
 
+import java.math.BigInteger;
+
 import function.LispFunction;
 import sexpression.*;
 
@@ -26,7 +28,7 @@ public class MINUS extends LispFunction {
             if (argRest.nullp()) {
                 // there is only one argument, so return the additive
                 // inverse of the number
-                return new LispNumber(-num1.getValue());
+                return new LispNumber(BigInteger.ZERO.subtract(num1.getValue()));
             }
 
             SExpression argSecond = argRest.getCar();
@@ -34,7 +36,7 @@ public class MINUS extends LispFunction {
             // make sure that the next argument is a number as well
             if (argSecond.numberp()) {
                 LispNumber num2 = (LispNumber) argSecond;
-                LispNumber difference = new LispNumber(num1.getValue() - num2.getValue());
+                LispNumber difference = new LispNumber(num1.getValue().subtract(num2.getValue()));
                 SExpression argCddr = argRest.getCdr();
 
                 if (argCddr.consp()) {
diff --git a/src/function/builtin/MULTIPLY.java b/src/function/builtin/MULTIPLY.java
index c20be32..d5cfcd3 100644
--- a/src/function/builtin/MULTIPLY.java
+++ b/src/function/builtin/MULTIPLY.java
@@ -1,5 +1,7 @@
 package function.builtin;
 
+import java.math.BigInteger;
+
 import function.LispFunction;
 import sexpression.*;
 
@@ -10,7 +12,7 @@ public class MULTIPLY extends LispFunction {
 
     public LispNumber call(Cons argList) {
         if (argList.nullp()) {
-            return new LispNumber(1);
+            return new LispNumber(BigInteger.ONE);
         }
 
         SExpression argFirst = argList.getCar();
@@ -20,7 +22,7 @@ public class MULTIPLY extends LispFunction {
             LispNumber num1 = (LispNumber) argFirst;
             LispNumber num2 = call(argRest);
 
-            return new LispNumber(num1.getValue() * num2.getValue());
+            return new LispNumber(num1.getValue().multiply(num2.getValue()));
         }
 
         throw new RuntimeException("*: " + argFirst + " is not a number");
diff --git a/src/function/builtin/PLUS.java b/src/function/builtin/PLUS.java
index 4d784bb..ce93494 100644
--- a/src/function/builtin/PLUS.java
+++ b/src/function/builtin/PLUS.java
@@ -1,5 +1,7 @@
 package function.builtin;
 
+import java.math.BigInteger;
+
 import function.LispFunction;
 import sexpression.*;
 
@@ -7,7 +9,7 @@ public class PLUS extends LispFunction {
 
     public LispNumber call(Cons argList) {
         if (argList.nullp()) {
-            return new LispNumber(0);
+            return new LispNumber(BigInteger.ZERO);
         }
 
         if (!argList.getCdr().listp())
@@ -20,7 +22,7 @@ public class PLUS extends LispFunction {
             LispNumber num1 = (LispNumber) argFirst;
             LispNumber num2 = call(argRest);
 
-            return new LispNumber(num1.getValue() + num2.getValue());
+            return new LispNumber(num1.getValue().add(num2.getValue()));
         }
 
         throw new RuntimeException("+: " + argFirst + " is not a number");
diff --git a/src/sexpression/LispNumber.java b/src/sexpression/LispNumber.java
index 7b122f4..36d0a6a 100644
--- a/src/sexpression/LispNumber.java
+++ b/src/sexpression/LispNumber.java
@@ -1,5 +1,6 @@
 package sexpression;
 
+import java.math.BigInteger;
 import java.text.MessageFormat;
 
 import error.LispException;
@@ -7,20 +8,20 @@ import error.LispException;
 @DisplayName("number")
 public class LispNumber extends Atom {
 
-    private int value;
+    private BigInteger value;
 
     public LispNumber(String text) {
         super(text.replaceFirst("^0+(?!$)", ""));
 
         try {
-            this.value = Integer.parseInt(text);
+            this.value = new BigInteger(text);
         } catch (NumberFormatException e) {
             throw new InvalidNumberException(text);
         }
     }
 
-    public LispNumber(int value) {
-        super(Integer.toString(value));
+    public LispNumber(BigInteger value) {
+        super(value.toString());
 
         this.value = value;
     }
@@ -29,7 +30,7 @@ public class LispNumber extends Atom {
         return true;
     }
 
-    public int getValue() {
+    public BigInteger getValue() {
         return value;
     }
 
diff --git a/test/function/UserDefinedFunctionTester.java b/test/function/UserDefinedFunctionTester.java
index 4b75e43..7546b74 100644
--- a/test/function/UserDefinedFunctionTester.java
+++ b/test/function/UserDefinedFunctionTester.java
@@ -42,7 +42,7 @@ public class UserDefinedFunctionTester {
     @Test
     public void oneArgumentFunction_ReturnsCorrectValue() {
         UserDefinedFunction function = createOneArgumentFunctionThatReturnsArgument();
-        SExpression argument = new LispNumber(23);
+        SExpression argument = new LispNumber("23");
         Cons argumentList = new Cons(argument, Nil.getUniqueInstance());
 
         assertSExpressionsMatch(argument, function.call(argumentList));
@@ -51,7 +51,7 @@ public class UserDefinedFunctionTester {
     @Test(expected = TooManyArgumentsException.class)
     public void oneArgumentFunction_ThrowsExceptionWithTooManyArguments() {
         UserDefinedFunction function = createOneArgumentFunctionThatReturnsArgument();
-        SExpression argument = new LispNumber(23);
+        SExpression argument = new LispNumber("23");
         Cons argumentList = new Cons(argument, new Cons(argument, Nil.getUniqueInstance()));
 
         function.call(argumentList);
diff --git a/test/sexpression/SExpressionTester.java b/test/sexpression/SExpressionTester.java
index c3535b4..014fa75 100644
--- a/test/sexpression/SExpressionTester.java
+++ b/test/sexpression/SExpressionTester.java
@@ -2,6 +2,8 @@ package sexpression;
 
 import static org.junit.Assert.*;
 
+import java.math.BigInteger;
+
 import org.junit.*;
 
 import error.ErrorManager;
@@ -35,7 +37,7 @@ public class SExpressionTester {
     public void testNumberValueToString() {
         String expected = "12";
 
-        assertSExpressionMatchesString(expected, new LispNumber(12));
+        assertSExpressionMatchesString(expected, new LispNumber("12"));
     }
 
     @Test
@@ -55,7 +57,7 @@ public class SExpressionTester {
     @Test
     public void testSimpleConsToString() {
         String expected = "(1)";
-        Cons cons = new Cons(new LispNumber(1), Nil.getUniqueInstance());
+        Cons cons = new Cons(new LispNumber("1"), Nil.getUniqueInstance());
 
         assertSExpressionMatchesString(expected, cons);
     }
@@ -63,7 +65,7 @@ public class SExpressionTester {
     @Test
     public void testComplexConsToString() {
         String expected = "(1 A \"string\")";
-        Cons list = new Cons(new LispNumber(1),
+        Cons list = new Cons(new LispNumber("1"),
                              new Cons(new Symbol("a"),
                                       new Cons(new LispString("\"string\""), Nil.getUniqueInstance())));
 
@@ -118,7 +120,7 @@ public class SExpressionTester {
     @Test
     public void afterSettingCarOfNil_ShouldStillBeNil() {
         Cons nil = Nil.getUniqueInstance();
-        nil.setCar(new LispNumber(2));
+        nil.setCar(new LispNumber("2"));
 
         assertEquals(nil.getCar(), Nil.getUniqueInstance());
     }
@@ -126,15 +128,15 @@ public class SExpressionTester {
     @Test
     public void afterSettingCdrOfNil_ShouldStillBeNil() {
         Cons nil = Nil.getUniqueInstance();
-        nil.setCdr(new LispNumber(2));
+        nil.setCdr(new LispNumber("2"));
 
         assertEquals(nil.getCdr(), Nil.getUniqueInstance());
     }
 
     @Test
     public void testNumberValue() {
-        int value = 12;
-        LispNumber number = new LispNumber(String.valueOf(value));
+        BigInteger value = new BigInteger("12");
+        LispNumber number = new LispNumber(value.toString());
 
         assertEquals(number.getValue(), value);
     }