Isolation is the degree to which a test can be independently run without side effects from other tests. JUnit has annotations and assumptions that help developers achieve test isolation. The @Test annotation tells JUnit that the public void method to which it is attached can be run as a test case. The @Before and @After annotations are used to set up and teardown conditions before and after each test case is run. These methods can be used to create or delete objects that are needed by the tests. By using these annotations, developers can achieve better test isolation and avoid having tests depend on each other. Assumptions provide a way for developers to express expectations about the environment in which their tests will be run. If an assumption fails, then the test will be aborted and reported as a failure. This helps to prevent false positives and ensures that tests are only run in environments where they are likely to succeed. By using assumptions, developers can further improve the isolation of their tests.