diff --git a/src/function/ArgumentValidator.java b/src/function/ArgumentValidator.java
index 115b3c2..545af26 100644
--- a/src/function/ArgumentValidator.java
+++ b/src/function/ArgumentValidator.java
@@ -173,22 +173,25 @@ public class ArgumentValidator {
         private static final long serialVersionUID = 1L;
         private String functionName;
         private String argument;
-        private Class<? extends SExpression> expected;
+        private Class<? extends SExpression> expectedType;
 
         public BadArgumentTypeException(String functionName, SExpression argument,
-                                        Class<? extends SExpression> expected) {
+                                        Class<? extends SExpression> expectedType) {
             this.functionName = functionName;
             this.argument = argument.toString();
-            this.expected = expected;
+            this.expectedType = expectedType;
         }
 
         @Override
         public String getMessage() {
-            DisplayName displayName = expected.getAnnotation(DisplayName.class);
-            String expectedType = (displayName == null) ? "unknown" : displayName.value();
-
             return MessageFormat.format("{0}: {1} is not the expected type of ''{2}''", functionName, argument,
-                                        expectedType);
+                                        getExpectedTypeName());
+        }
+
+        private String getExpectedTypeName() {
+            DisplayName displayName = expectedType.getAnnotation(DisplayName.class);
+
+            return (displayName == null) ? "unknown" : displayName.value();
         }
     }
 
diff --git a/src/function/UserDefinedFunction.java b/src/function/UserDefinedFunction.java
index b276dde..a9fc27b 100644
--- a/src/function/UserDefinedFunction.java
+++ b/src/function/UserDefinedFunction.java
@@ -38,9 +38,7 @@ public class UserDefinedFunction extends LispFunction {
         // bind the values of the arguments to the formal parameter names
         for (String param : formalParameters) {
             SExpression currentArg = argList.getCar();
-
             environment.put(param, currentArg);
-
             argList = (Cons) argList.getCdr();
         }
 
diff --git a/src/function/builtin/CONS.java b/src/function/builtin/CONS.java
index 2913115..1703ec8 100644
--- a/src/function/builtin/CONS.java
+++ b/src/function/builtin/CONS.java
@@ -1,38 +1,25 @@
 package function.builtin;
 
-import function.LispFunction;
+import function.*;
 import sexpression.*;
 
-/**
- * <code>CONS</code> represents the CONS function in Lisp.
- */
 public class CONS extends LispFunction {
 
-    // The number of arguments that CONS takes.
-    private static final int NUM_ARGS = 2;
+    private ArgumentValidator argumentValidator;
 
-    public Cons call(Cons argList) {
-        // retrieve the number of arguments passed to CONS
-        int argListLength = LENGTH.getLength(argList);
+    public CONS() {
+        this.argumentValidator = new ArgumentValidator("CONS");
+        this.argumentValidator.setExactNumberOfArguments(2);
+    }
 
-        // make sure we have received the proper number of arguments
-        if (argListLength != NUM_ARGS) {
-            Cons originalSExpr = new Cons(new Symbol("CONS"), argList);
-            String errMsg = "too " + ((argListLength > NUM_ARGS) ? "many" : "few") + " arguments given to CONS: "
-                            + originalSExpr;
+    public Cons call(Cons argumentList) {
+        argumentValidator.validate(argumentList);
 
-            throw new RuntimeException(errMsg);
-        }
+        Cons cdr = (Cons) argumentList.getCdr();
+        SExpression firstArgument = argumentList.getCar();
+        SExpression secondArgument = cdr.getCar();
 
-        // the car of the CONS cell we are going to create
-        SExpression argOne = argList.getCar();
-
-        Cons cdr = (Cons) argList.getCdr();
-
-        // the cdr of the CONS cell we are going to create
-        SExpression argTwo = cdr.getCar();
-
-        return new Cons(argOne, argTwo);
+        return new Cons(firstArgument, secondArgument);
     }
 
 }
diff --git a/src/function/builtin/DEFUN.java b/src/function/builtin/DEFUN.java
index ad167eb..6498b84 100644
--- a/src/function/builtin/DEFUN.java
+++ b/src/function/builtin/DEFUN.java
@@ -5,34 +5,21 @@ import java.util.HashMap;
 import function.*;
 import sexpression.*;
 
-/**
- * <code>DEFUN</code> represents the DEFUN form in Lisp.
- */
 public class DEFUN extends LispFunction {
 
-    // The minimum number of arguments that DEFUN takes.
-    private static final int MIN_ARGS = 3;
+    private ArgumentValidator argumentValidator;
 
-    public SExpression call(Cons argList) {
-        // retrieve the number of arguments passed to DEFUN
-        int argListLength = LENGTH.getLength(argList);
+    public DEFUN() {
+        this.argumentValidator = new ArgumentValidator("DEFUN");
+        this.argumentValidator.setMinimumNumberOfArguments(3);
+        this.argumentValidator.setFirstArgumentExpectedType(Symbol.class);
+    }
 
-        // make sure we have received the proper number of arguments
-        if (argListLength < MIN_ARGS) {
-            Cons originalSExpr = new Cons(new Symbol("DEFUN"), argList);
-            String errMsg = "too few arguments given to DEFUN: " + originalSExpr;
+    public SExpression call(Cons argumentList) {
+        argumentValidator.validate(argumentList);
 
-            throw new RuntimeException(errMsg);
-        }
-
-        SExpression name = argList.getCar(); // name of the function
-
-        // make sure the function name is a symbol
-        if (!name.symbolp()) {
-            throw new RuntimeException("DEFUN: " + name + " is not a symbol");
-        }
-
-        Cons cdr = (Cons) argList.getCdr();
+        Cons cdr = (Cons) argumentList.getCdr();
+        SExpression name = argumentList.getCar(); // name of the function
         SExpression cadr = cdr.getCar();
 
         // make sure the list of arguments (lambda list) is a proper list
@@ -60,11 +47,6 @@ public class DEFUN extends LispFunction {
         return name;
     }
 
-    /**
-     * Determine if the arguments passed to this Lisp function should be evaluated.
-     *
-     * @return <code>false</code>
-     */
     public boolean evaluateArguments() {
         return false;
     }
diff --git a/test/function/builtin/CARTester.java b/test/function/builtin/CARTester.java
index d084e7d..e619370 100644
--- a/test/function/builtin/CARTester.java
+++ b/test/function/builtin/CARTester.java
@@ -36,16 +36,12 @@ public class CARTester {
 
     @Test(expected = TooManyArgumentsException.class)
     public void testCarWithTooManyArguments() {
-        String input = "(car '(1 2) '(1 2) \"oh\")";
-
-        assertSExpressionsMatch(evaluateString(input), parseString("1"));
+        evaluateString("(car '(1 2) '(1 2) \"oh\")");
     }
 
     @Test(expected = TooFewArgumentsException.class)
     public void testCarWithTooFewArguments() {
-        String input = "(car)";
-
-        assertSExpressionsMatch(evaluateString(input), parseString("1"));
+        evaluateString("(car)");
     }
 
 }
diff --git a/test/function/builtin/CDRTester.java b/test/function/builtin/CDRTester.java
index 7fec962..36b135c 100644
--- a/test/function/builtin/CDRTester.java
+++ b/test/function/builtin/CDRTester.java
@@ -36,16 +36,12 @@ public class CDRTester {
 
     @Test(expected = TooManyArgumentsException.class)
     public void testCdrWithTooManyArguments() {
-        String input = "(cdr '(1 2) '(1 2) \"oh\")";
-
-        assertSExpressionsMatch(evaluateString(input), parseString("1"));
+        evaluateString("(cdr '(1 2) '(1 2) \"oh\")");
     }
 
     @Test(expected = TooFewArgumentsException.class)
-        public void testCdrWithTooFewArguments() {
-            String input = "(cdr)";
-
-        assertSExpressionsMatch(evaluateString(input), parseString("1"));
+    public void testCdrWithTooFewArguments() {
+        evaluateString("(cdr)");
     }
 
 }
diff --git a/test/function/builtin/CONSTester.java b/test/function/builtin/CONSTester.java
new file mode 100644
index 0000000..3ad96b5
--- /dev/null
+++ b/test/function/builtin/CONSTester.java
@@ -0,0 +1,61 @@
+package function.builtin;
+
+import static testutil.TestUtilities.*;
+
+import org.junit.Test;
+
+import function.ArgumentValidator.*;
+import sexpression.*;
+
+public class CONSTester {
+
+    @Test
+    public void testConsWithNilValues() {
+        String input = "(cons () nil)";
+
+        assertSExpressionsMatch(evaluateString(input), parseString("(())"));
+    }
+
+    @Test
+    public void testConsWithTwoSymbols() {
+        String input = "(cons 'a 'b)";
+
+        assertSExpressionsMatch(evaluateString(input), new Cons(new Symbol("A"), new Symbol("B")));
+    }
+
+    @Test
+    public void testConsWithListAsCdr() {
+        String input = "(cons 1 '(2 3))";
+
+        assertSExpressionsMatch(evaluateString(input), parseString("(1 2 3)"));
+    }
+    @Test
+    public void testConsWithTwoLists() {
+        String input = "(cons '(1 2) '(3 4))";
+
+        assertSExpressionsMatch(evaluateString(input), parseString("((1 2) 3 4)"));
+    }
+
+
+    @Test
+    public void testConsWithList() {
+        String input = "(cons nil '(2 3))";
+
+        assertSExpressionsMatch(evaluateString(input), parseString("(nil 2 3)"));
+    }
+
+    @Test(expected = TooManyArgumentsException.class)
+    public void testConsWithTooManyArguments() {
+        String input = "(cons 1 2 3)";
+
+        evaluateString(input);
+    }
+
+    @Test(expected = TooFewArgumentsException.class)
+    public void testConsWithTooFewArguments() {
+        String input = "(cons 1)";
+
+        evaluateString(input);
+    }
+
+}
diff --git a/test/function/builtin/DEFUNTester.java b/test/function/builtin/DEFUNTester.java
new file mode 100644
index 0000000..1318a45
--- /dev/null
+++ b/test/function/builtin/DEFUNTester.java
@@ -0,0 +1,29 @@
+package function.builtin;
+
+import static testutil.TestUtilities.*;
+
+import org.junit.Test;
+
+import function.ArgumentValidator.*;
+
+public class DEFUNTester {
+
+    @Test
+    public void testDefun() {
+        String input = "(defun f () nil)";
+
+        assertSExpressionsMatch(evaluateString(input), parseString("f"));
+        assertSExpressionsMatch(evaluateString("(f)"), parseString("()"));
+    }
+
+    @Test(expected = BadArgumentTypeException.class)
+    public void testDefunWithNonSymbolName() {
+        evaluateString("(defun 1 () ())");
+    }
+
+    @Test(expected = TooFewArgumentsException.class)
+    public void testApplyWithTooFewArguments() {
+        evaluateString("(defun 1 ())");
+    }
+
+}