diff --git a/src/function/builtin/special/DEFUN.java b/src/function/builtin/special/DEFUN.java
index 6f1a86a..75c1dc0 100644
--- a/src/function/builtin/special/DEFUN.java
+++ b/src/function/builtin/special/DEFUN.java
@@ -1,7 +1,9 @@
 package function.builtin.special;
 
+import java.text.MessageFormat;
 import java.util.HashMap;
 
+import environment.Environment;
 import function.*;
 import function.builtin.EVAL;
 import function.builtin.cons.LIST;
@@ -12,6 +14,7 @@ public class DEFUN extends LispFunction {
     private ArgumentValidator argumentValidator;
     private ArgumentValidator lambdaListIsListValidator;
     private ArgumentValidator lambdaListValidator;
+    private Environment environment;
 
     public DEFUN() {
         this.argumentValidator = new ArgumentValidator("DEFUN");
@@ -23,6 +26,8 @@ public class DEFUN extends LispFunction {
 
         this.lambdaListValidator = new ArgumentValidator("DEFUN|lambda_list|");
         this.lambdaListValidator.setEveryArgumentExpectedType(Symbol.class);
+
+        this.environment = Environment.getInstance();
     }
 
     public SExpression call(Cons argumentList) {
@@ -37,16 +42,26 @@ public class DEFUN extends LispFunction {
         lambdaListValidator.validate(lambdaList);
 
         Cons functionBody = (Cons) remainingArguments.getCdr();
+        UserDefinedFunction function = new UserDefinedFunction(functionName.toString(), lambdaList, functionBody);
         HashMap<String, LispFunction> functionTable = EVAL.getFunctionTable();
 
-        if (functionTable.containsKey(functionName.toString()))
-            System.out.println("WARNING: redefining function " + functionName.toString());
+        if (isAlreadyDefined(functionName, functionTable))
+            printWarning(functionName);
 
-        functionTable.put(functionName.toString(), new UserDefinedFunction(functionName.toString(), lambdaList, functionBody));
+        functionTable.put(functionName.toString(), function);
 
         return functionName;
     }
 
+    private boolean isAlreadyDefined(SExpression functionName, HashMap<String, LispFunction> functionTable) {
+        return functionTable.containsKey(functionName.toString());
+    }
+
+    private void printWarning(SExpression functionName) {
+        String message = MessageFormat.format("WARNING: redefining function {0}", functionName.toString());
+        environment.getOutput().println(message);
+    }
+
     public boolean evaluateArguments() {
         return false;
     }
diff --git a/test/function/builtin/special/DEFUNTester.java b/test/function/builtin/special/DEFUNTester.java
index 4705fd2..ae6ba24 100644
--- a/test/function/builtin/special/DEFUNTester.java
+++ b/test/function/builtin/special/DEFUNTester.java
@@ -1,13 +1,29 @@
 package function.builtin.special;
 
+import static org.junit.Assert.assertEquals;
 import static testutil.TestUtilities.*;
 
-import org.junit.Test;
+import java.io.*;
 
+import org.junit.*;
+
+import environment.Environment;
 import function.ArgumentValidator.*;
 
 public class DEFUNTester {
 
+    private ByteArrayOutputStream outputStream;
+
+    private void assertPrinted(String expected) {
+        assertEquals(expected, outputStream.toString());
+    }
+
+    @Before
+    public void setUp() {
+        this.outputStream = new ByteArrayOutputStream();
+        Environment.getInstance().setOutput(new PrintStream(outputStream));
+    }
+
     @Test
     public void testDefun() {
         String input = "(defun f () nil)";
@@ -16,6 +32,24 @@ public class DEFUNTester {
         assertSExpressionsMatch(parseString("()"), evaluateString("(f)"));
     }
 
+    @Test
+    public void redefineFunction_DisplaysWarning() {
+        String input = "(defun myFunction () nil)";
+        evaluateString(input);
+        evaluateString(input);
+
+        assertPrinted("WARNING: redefining function MYFUNCTION\n");
+    }
+
+    @Test
+    public void redefineFunction_ActuallyRedefinesFunction() {
+        evaluateString("(defun myFunction2 () nil)");
+        evaluateString("(defun myFunction2 () T)");
+
+        assertPrinted("WARNING: redefining function MYFUNCTION2\n");
+        assertSExpressionsMatch(parseString("t"), evaluateString("(myFunction2)"));
+    }
+
     @Test(expected = DottedArgumentListException.class)
     public void testDefunWithDottedLambdaList() {
         String input = "(funcall 'defun 'x (cons 'a 'b) ())";