Mobile apps testing is becoming increasingly important as mobile takes over the world. Appium is a cross-platform solution for automating native and hybrid mobile app testing using the WebDriver protocol. It allows testing the same app submitted to app stores by writing tests in any language using any framework. Appium acts as an HTTP server, starting a test case on the device that handles WebDriver commands, proxying them to the appropriate automation tools for each platform like UIAutomation for iOS and UiAutomator for Android. This enables cross-platform mobile testing with one test across two platforms.