TestCase cannot be resolved to a type

This means that your project isn't setup to include the JUnit libraries when it compiles; JUnit is not included in the Java runtime libraries (JRE System Library) so you have to add it to the build path.

The Long Way

To add the JUnit libraries to the Build Path, right-click on your project in the Package Explorer and select "Properties":

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You are then taken to a Project Properties Configuration window. Select "Java Build Path" (from the left) and you should see four tabs; "Source", "Projects", "Libraries" and "Order and Export". Select the "Libraries" tab.

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Click on "Add External JARs" and add the junit.jar libray found at:

/nfs/encs/ArchDep/i686.linux26-RHEL5/pkg/eclipse-jee.galileo/root/eclipse/plugins/org.junit_3.8.2.v20090203-1005/junit.jar

Click OK and return to the workbench.

Why "Add External JARs" and Not "Add JARs"?

Since the jar file you're adding to the build path is not already in the directory structure of your workbench, it is considered an external JAR.

The Short Way

Open (or werite from scratch) a .java file which contains a class extending junit.framework.TestCase. On the line which declares the class (public class MyClass extends TestCase) you'll see an Eclipse Quick Fix light bulb:

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Click on it and select Add JUnit 3 library to the build path and Eclipse will do all of the above much more painlessly for you.