How do you make Selenium tests run in parallel safely?

Parallelism is a runner feature that opens several browser sessions at once. The suite must be safe for concurrent execution: each session needs its own driver, each test needs data it owns, and shared external resources need coordination.

public abstract class BaseTest {
    private static final ThreadLocal<WebDriver> DRIVER = new ThreadLocal<>();

    protected WebDriver driver() {
        return DRIVER.get();
    }

    @BeforeMethod
    public void setUp() {
        DRIVER.set(DriverFactory.create());
    }

    @AfterMethod(alwaysRun = true)
    public void tearDown() {
        if (DRIVER.get() != null) {
            DRIVER.get().quit();
            DRIVER.remove();
        }
    }
}

• TestNG can run methods or classes in parallel with a configured thread count
• JUnit 5 uses parallel execution configuration and a per-test extension
• ThreadLocal prevents two threads from sharing a browser session
• Generate unique users, orders and file names per test rather than using fixed fixtures
• Avoid tests that depend on execution order or on another test's data
• Size thread count against Grid slots, CPU and database capacity

A suite can pass serially and fail in parallel because of hidden coupling. Login with the same account in two sessions can invalidate tokens, fixed file paths collide, and a shared database row can be deleted by another test. Run the parallel suite early and often rather than enabling it at the end.