Running tests one by one can be slow, especially as your test suite grows. Parallel execution lets you run multiple tests at the same time, which cuts down total time.

Tools like pytest-xdist (Python), TestNG (Java), or Cypress’s parallel features make it easy. You can also use Selenium Grid or services like BrowserStack or Sauce Labs for parallel runs across browsers.

To get this right, you need independent tests. Don’t let one test depend on another. Don’t share global states or login sessions across tests. Keep each test self-contained.

Also, manage your resources. If you're using a cloud grid, track how many parallel sessions you're allowed. On your own machine, make sure your CPU and memory can handle it.

Parallel testing isn’t just faster—it helps reveal flaky tests too. A test that fails only when run next to others usually has hidden issues. Running tests this way helps improve quality.