Test isolation is a crucial best practice in software testing that ensures tests are run independently, preventing one test's state from affecting another and thereby ensuring reliable and reproducible results. With Cypress version 12, test isolation is enforced by default, resetting the test and browser state before each test to clear any potential interference from previous tests, including the DOM state, cookies, and local and session storage. This approach not only enhances the reliability of test results by eliminating dependencies between tests, but also allows for parallelization within CI/CD pipelines, significantly improving test execution time. Techniques such as using beforeEach hooks to define shared setups and the cy.session() command for preserving login sessions are recommended to maintain test independence while optimizing performance. While disabling test isolation can improve performance for certain test suites, it introduces risks of misleading failures and increased complexity in debugging, highlighting the importance of maintaining isolated tests to ensure a reliable and scalable testing framework.