diff --git a/src/function/builtin/MINUS.java b/src/function/builtin/MINUS.java
index d9b4165..9bace55 100644
--- a/src/function/builtin/MINUS.java
+++ b/src/function/builtin/MINUS.java
@@ -2,54 +2,46 @@ package function.builtin;
 
 import java.math.BigInteger;
 
-import function.LispFunction;
+import function.*;
 import sexpression.*;
 
-/**
- * <code>MINUS</code> represents the '-' function in Lisp.
- */
 public class MINUS extends LispFunction {
 
-    public SExpression call(Cons argList) {
-        // make sure we have received at least one argument
-        if (argList.nullp()) {
-            Cons originalSExpr = new Cons(new Symbol("-"), argList);
+    private ArgumentValidator argumentValidator;
 
-            throw new RuntimeException("too few arguments given to -: " + originalSExpr);
-        }
+    public MINUS() {
+        this.argumentValidator = new ArgumentValidator("-");
+        this.argumentValidator.setMinimumNumberOfArguments(1);
+        this.argumentValidator.setEveryArgumentExpectedType(LispNumber.class);
+    }
 
-        SExpression argFirst = argList.getCar();
-        Cons argRest = (Cons) argList.getCdr();
+    public SExpression call(Cons argumentList) {
+        argumentValidator.validate(argumentList);
 
-        // make sure that the first argument is a number
-        if (argFirst.numberp()) {
-            LispNumber num1 = (LispNumber) argFirst;
+        return callTailRecursive(argumentList);
+    }
 
-            if (argRest.nullp()) {
-                // there is only one argument, so return the additive
-                // inverse of the number
-                return new LispNumber(BigInteger.ZERO.subtract(num1.getValue()));
-            }
+    private SExpression callTailRecursive(Cons argumentList) {
+        Cons remainingArguments = (Cons) argumentList.getCdr();
+        SExpression firstArgument = argumentList.getCar();
+        LispNumber number1 = (LispNumber) firstArgument;
 
-            SExpression argSecond = argRest.getCar();
+        if (remainingArguments.nullp())
+            return additiveInverse(number1);
 
-            // make sure that the next argument is a number as well
-            if (argSecond.numberp()) {
-                LispNumber num2 = (LispNumber) argSecond;
-                LispNumber difference = new LispNumber(num1.getValue().subtract(num2.getValue()));
-                SExpression argCddr = argRest.getCdr();
+        SExpression secondArgument = remainingArguments.getCar();
+        LispNumber number2 = (LispNumber) secondArgument;
+        LispNumber difference = new LispNumber(number1.getValue().subtract(number2.getValue()));
+        SExpression remainingNumbers = remainingArguments.getCdr();
 
-                if (argCddr.consp()) {
-                    return call(new Cons(difference, argCddr));
-                }
+        if (!remainingNumbers.consp())
+            return difference;
 
-                return difference;
-            }
+        return callTailRecursive(new Cons(difference, remainingNumbers));
+    }
 
-            throw new RuntimeException("-: " + argSecond + " is not a number");
-        }
-
-        throw new RuntimeException("-: " + argFirst + " is not a number");
+    private LispNumber additiveInverse(LispNumber number) {
+        return new LispNumber(BigInteger.ZERO.subtract(number.getValue()));
     }
 
 }
diff --git a/src/function/builtin/MULTIPLY.java b/src/function/builtin/MULTIPLY.java
index d5cfcd3..eceb095 100644
--- a/src/function/builtin/MULTIPLY.java
+++ b/src/function/builtin/MULTIPLY.java
@@ -1,31 +1,40 @@
 package function.builtin;
 
-import java.math.BigInteger;
-
-import function.LispFunction;
+import function.*;
 import sexpression.*;
 
-/**
- * <code>MULTIPLY</code> represents the '*' function in Lisp.
- */
 public class MULTIPLY extends LispFunction {
 
-    public LispNumber call(Cons argList) {
-        if (argList.nullp()) {
-            return new LispNumber(BigInteger.ONE);
-        }
+    private ArgumentValidator argumentValidator;
 
-        SExpression argFirst = argList.getCar();
-        Cons argRest = (Cons) argList.getCdr();
+    public MULTIPLY() {
+        this.argumentValidator = new ArgumentValidator("*");
+        this.argumentValidator.setEveryArgumentExpectedType(LispNumber.class);
+    }
 
-        if (argFirst.numberp()) {
-            LispNumber num1 = (LispNumber) argFirst;
-            LispNumber num2 = call(argRest);
+    public LispNumber call(Cons argumentList) {
+        argumentValidator.validate(argumentList);
 
-            return new LispNumber(num1.getValue().multiply(num2.getValue()));
-        }
+        return callTailRecursive(new Cons(LispNumber.ONE, argumentList));
+    }
 
-        throw new RuntimeException("*: " + argFirst + " is not a number");
+    private LispNumber callTailRecursive(Cons argumentList) {
+        Cons remainingArguments = (Cons) argumentList.getCdr();
+        SExpression firstArgument = argumentList.getCar();
+        LispNumber number1 = (LispNumber) firstArgument;
+
+        if (remainingArguments.nullp())
+            return number1;
+
+        SExpression secondArgument = remainingArguments.getCar();
+        LispNumber number2 = (LispNumber) secondArgument;
+        LispNumber product = new LispNumber(number1.getValue().multiply(number2.getValue()));
+        SExpression remainingNumbers = remainingArguments.getCdr();
+
+        if (!remainingNumbers.consp())
+            return product;
+
+        return callTailRecursive(new Cons(product, remainingNumbers));
     }
 
 }
diff --git a/src/function/builtin/NULL.java b/src/function/builtin/NULL.java
index 50dc455..650858f 100644
--- a/src/function/builtin/NULL.java
+++ b/src/function/builtin/NULL.java
@@ -1,32 +1,21 @@
 package function.builtin;
 
-import function.LispFunction;
+import function.*;
 import sexpression.*;
 
-/**
- * <code>NULL</code> represents the NULL function in Lisp.
- */
 public class NULL extends LispFunction {
 
-    // The number of arguments that NULL takes.
-    private static final int NUM_ARGS = 1;
+    private ArgumentValidator argumentValidator;
 
-    public SExpression call(Cons argList) {
-        // retrieve the number of arguments passed to NULL
-        int argListLength = LENGTH.getLength(argList);
+    public NULL() {
+        this.argumentValidator = new ArgumentValidator("NULL");
+        this.argumentValidator.setExactNumberOfArguments(1);
+    }
 
-        // make sure we have received the proper number of arguments
-        if (argListLength != NUM_ARGS) {
-            Cons originalSExpr = new Cons(new Symbol("NULL"), argList);
-            String errMsg = "too " + ((argListLength > NUM_ARGS) ? "many" : "few") + " arguments given to NULL: "
-                            + originalSExpr;
+    public SExpression call(Cons argumentList) {
+        argumentValidator.validate(argumentList);
 
-            throw new RuntimeException(errMsg);
-        }
-
-        SExpression arg = argList.getCar();
-
-        return (arg.nullp() ? Symbol.T : Nil.getInstance());
+        return argumentList.getCar().nullp() ? Symbol.T : Nil.getInstance();
     }
 
 }
diff --git a/src/function/builtin/PLUS.java b/src/function/builtin/PLUS.java
index ce93494..db24685 100644
--- a/src/function/builtin/PLUS.java
+++ b/src/function/builtin/PLUS.java
@@ -1,31 +1,40 @@
 package function.builtin;
 
-import java.math.BigInteger;
-
-import function.LispFunction;
+import function.*;
 import sexpression.*;
 
 public class PLUS extends LispFunction {
 
-    public LispNumber call(Cons argList) {
-        if (argList.nullp()) {
-            return new LispNumber(BigInteger.ZERO);
-        }
+    private ArgumentValidator argumentValidator;
 
-        if (!argList.getCdr().listp())
-            throw new RuntimeException("+: " + argList + " is dotted");
+    public PLUS() {
+        this.argumentValidator = new ArgumentValidator("+");
+        this.argumentValidator.setEveryArgumentExpectedType(LispNumber.class);
+    }
 
-        SExpression argFirst = argList.getCar();
-        Cons argRest = (Cons) argList.getCdr();
+    public LispNumber call(Cons argumentList) {
+        argumentValidator.validate(argumentList);
 
-        if (argFirst.numberp()) {
-            LispNumber num1 = (LispNumber) argFirst;
-            LispNumber num2 = call(argRest);
+        return callTailRecursive(new Cons(LispNumber.ZERO, argumentList));
+    }
 
-            return new LispNumber(num1.getValue().add(num2.getValue()));
-        }
+    private LispNumber callTailRecursive(Cons argumentList) {
+        Cons remainingArguments = (Cons) argumentList.getCdr();
+        SExpression firstArgument = argumentList.getCar();
+        LispNumber number1 = (LispNumber) firstArgument;
 
-        throw new RuntimeException("+: " + argFirst + " is not a number");
+        if (remainingArguments.nullp())
+            return number1;
+
+        SExpression secondArgument = remainingArguments.getCar();
+        LispNumber number2 = (LispNumber) secondArgument;
+        LispNumber sum = new LispNumber(number1.getValue().add(number2.getValue()));
+        SExpression remainingNumbers = remainingArguments.getCdr();
+
+        if (!remainingNumbers.consp())
+            return sum;
+
+        return callTailRecursive(new Cons(sum, remainingNumbers));
     }
 
 }
diff --git a/src/sexpression/LispNumber.java b/src/sexpression/LispNumber.java
index 36d0a6a..6ceb8b9 100644
--- a/src/sexpression/LispNumber.java
+++ b/src/sexpression/LispNumber.java
@@ -7,6 +7,9 @@ import error.LispException;
 
 @DisplayName("number")
 public class LispNumber extends Atom {
+    
+    public static final LispNumber ZERO = new LispNumber(BigInteger.ZERO);
+    public static final LispNumber ONE = new LispNumber(BigInteger.ONE);
 
     private BigInteger value;
 
diff --git a/test/function/builtin/LISTPTester.java b/test/function/builtin/LISTPTester.java
index 48bab95..de9f12b 100644
--- a/test/function/builtin/LISTPTester.java
+++ b/test/function/builtin/LISTPTester.java
@@ -24,16 +24,12 @@ public class LISTPTester {
 
     @Test(expected = TooFewArgumentsException.class)
     public void testListpWithTooFewArguments() {
-        String input = "(listp)";
-
-        assertSExpressionsMatch(evaluateString(input), parseString("NIL"));
+        evaluateString("(listp)");
     }
 
     @Test(expected = TooManyArgumentsException.class)
     public void testListpWithTooManyArguments() {
-        String input = "(listp '() '())";
-
-        assertSExpressionsMatch(evaluateString(input), parseString("NIL"));
+        evaluateString("(listp '() '())");
     }
 
 }
diff --git a/test/function/builtin/MINUSTester.java b/test/function/builtin/MINUSTester.java
new file mode 100644
index 0000000..1bc7711
--- /dev/null
+++ b/test/function/builtin/MINUSTester.java
@@ -0,0 +1,50 @@
+package function.builtin;
+
+import static testutil.TestUtilities.*;
+
+import org.junit.Test;
+
+import function.ArgumentValidator.*;
+import sexpression.LispNumber;
+
+public class MINUSTester {
+
+    @Test
+    public void testMinusWithOneNumber() {
+        String input = "(- 27)";
+
+        assertSExpressionsMatch(evaluateString(input), new LispNumber("-27"));
+    }
+
+    @Test
+    public void testMinusWithTwoNumbers() {
+        String input = "(- 5 3)";
+
+        assertSExpressionsMatch(evaluateString(input), new LispNumber("2"));
+    }
+
+    @Test
+    public void testMinusWithManyNumbers_PositiveResult() {
+        String input = "(- 200 100 10 5)";
+
+        assertSExpressionsMatch(evaluateString(input), new LispNumber("85"));
+    }
+
+    @Test
+    public void testMinusWithManyNumbers_NegativeResult() {
+        String input = "(- 100 200 20 5)";
+
+        assertSExpressionsMatch(evaluateString(input), new LispNumber("-125"));
+    }
+
+    @Test(expected = BadArgumentTypeException.class)
+    public void testMinusWithNonNumber() {
+        evaluateString("(- 'a 'b)");
+    }
+
+    @Test(expected = TooFewArgumentsException.class)
+    public void testMinusWithTooFewArguments() {
+        evaluateString("(-)");
+    }
+
+}
diff --git a/test/function/builtin/MULTIPLYTester.java b/test/function/builtin/MULTIPLYTester.java
new file mode 100644
index 0000000..f17e24c
--- /dev/null
+++ b/test/function/builtin/MULTIPLYTester.java
@@ -0,0 +1,52 @@
+package function.builtin;
+
+import static testutil.TestUtilities.*;
+
+import org.junit.Test;
+
+import function.ArgumentValidator.BadArgumentTypeException;
+import sexpression.LispNumber;
+
+public class MULTIPLYTester {
+
+    @Test
+    public void testMultiplyWithNoArguments() {
+        String input = "(*)";
+
+        assertSExpressionsMatch(evaluateString(input), new LispNumber("1"));
+    }
+
+    @Test
+    public void testMultiplyWithOneNumber() {
+        String input = "(* 8)";
+
+        assertSExpressionsMatch(evaluateString(input), new LispNumber("8"));
+    }
+
+    @Test
+    public void testMultiplyWithTwoNumbers() {
+        String input = "(* 5 3)";
+
+        assertSExpressionsMatch(evaluateString(input), new LispNumber("15"));
+    }
+
+    @Test
+    public void testMultiplyWithManyNumbers_PositiveResult() {
+        String input = "(* 2 3 5 1)";
+
+        assertSExpressionsMatch(evaluateString(input), new LispNumber("30"));
+    }
+
+    @Test
+    public void testMultiplyWithManyNumbers_NegativeResult() {
+        String input = "(* 3 (- 2) 10 2)";
+
+        assertSExpressionsMatch(evaluateString(input), new LispNumber("-120"));
+    }
+
+    @Test(expected = BadArgumentTypeException.class)
+    public void testMultiplyWithNonNumber() {
+        evaluateString("(* 'a 'b)");
+    }
+
+}
diff --git a/test/function/builtin/NULLTester.java b/test/function/builtin/NULLTester.java
new file mode 100644
index 0000000..5a0198a
--- /dev/null
+++ b/test/function/builtin/NULLTester.java
@@ -0,0 +1,35 @@
+package function.builtin;
+
+import static testutil.TestUtilities.*;
+
+import org.junit.Test;
+
+import function.ArgumentValidator.*;
+
+public class NULLTester {
+
+    @Test
+    public void testNilIsNull() {
+        String input = "(null ())";
+
+        assertSExpressionsMatch(evaluateString(input), parseString("T"));
+    }
+
+    @Test
+    public void testListIsNotNull() {
+        String input = "(null '(1))";
+
+        assertSExpressionsMatch(evaluateString(input), parseString("NIL"));
+    }
+
+    @Test(expected = TooFewArgumentsException.class)
+    public void testNullWithTooFewArguments() {
+        evaluateString("(null)");
+    }
+
+    @Test(expected = TooManyArgumentsException.class)
+    public void testNullWithTooManyArguments() {
+        evaluateString("(null 1 2)");
+    }
+
+}
diff --git a/test/function/builtin/PLUSTester.java b/test/function/builtin/PLUSTester.java
new file mode 100644
index 0000000..96b6c38
--- /dev/null
+++ b/test/function/builtin/PLUSTester.java
@@ -0,0 +1,52 @@
+package function.builtin;
+
+import static testutil.TestUtilities.*;
+
+import org.junit.Test;
+
+import function.ArgumentValidator.BadArgumentTypeException;
+import sexpression.LispNumber;
+
+public class PLUSTester {
+
+    @Test
+    public void testPlusWithNoArguments() {
+        String input = "(+)";
+
+        assertSExpressionsMatch(evaluateString(input), new LispNumber("0"));
+    }
+
+    @Test
+    public void testPlusWithOneNumber() {
+        String input = "(+ 27)";
+
+        assertSExpressionsMatch(evaluateString(input), new LispNumber("27"));
+    }
+
+    @Test
+    public void testPlusWithTwoNumbers() {
+        String input = "(+ 5 3)";
+
+        assertSExpressionsMatch(evaluateString(input), new LispNumber("8"));
+    }
+
+    @Test
+    public void testPlusWithManyNumbers_PositiveResult() {
+        String input = "(+ 200 100 10 5)";
+
+        assertSExpressionsMatch(evaluateString(input), new LispNumber("315"));
+    }
+
+    @Test
+    public void testPlusWithManyNumbers_NegativeResult() {
+        String input = "(+ 100 (- 200) 20 5)";
+
+        assertSExpressionsMatch(evaluateString(input), new LispNumber("-75"));
+    }
+
+    @Test(expected = BadArgumentTypeException.class)
+    public void testPlusWithNonNumber() {
+        evaluateString("(+ 'a 'b)");
+    }
+
+}
diff --git a/test/sexpression/SExpressionTester.java b/test/sexpression/SExpressionTester.java
index 4e9a332..6f5a786 100644
--- a/test/sexpression/SExpressionTester.java
+++ b/test/sexpression/SExpressionTester.java
@@ -66,8 +66,7 @@ public class SExpressionTester {
     public void testComplexConsToString() {
         String expected = "(1 A \"string\")";
         Cons list = new Cons(new LispNumber("1"),
-                             new Cons(new Symbol("a"),
-                                      new Cons(new LispString("\"string\""), Nil.getInstance())));
+                             new Cons(new Symbol("a"), new Cons(new LispString("\"string\""), Nil.getInstance())));
 
         assertSExpressionMatchesString(expected, list);
     }
@@ -99,10 +98,8 @@ public class SExpressionTester {
     @Test
     public void testLambdaExpressionGetFunction() {
         String expected = "(LAMBDA)";
-        UserDefinedFunction function = new UserDefinedFunction(expected, Nil.getInstance(),
-                                                               Nil.getInstance());
-        LambdaExpression lambda = new LambdaExpression(new Cons(new Symbol("lambda"), Nil.getInstance()),
-                                                       function);
+        UserDefinedFunction function = new UserDefinedFunction(expected, Nil.getInstance(), Nil.getInstance());
+        LambdaExpression lambda = new LambdaExpression(new Cons(new Symbol("lambda"), Nil.getInstance()), function);
 
         assertEquals(function, lambda.getFunction());
     }
@@ -167,4 +164,9 @@ public class SExpressionTester {
         }
     }
 
+    @Test
+    public void testLispNumberConstants() {
+        assertEquals(LispNumber.ONE.getValue(), new BigInteger("1"));
+    }
+
 }