Selenium is an open source web test tool that runs in the browser. It can be run directly in most modern web browsers. Selenium comes in three formats selenium core, IntegratedDevelopmentEnvironment and RemoteControl. IDE comes as a Firefox plug-in where as in RC; we have server and client programs. We can convert the generated scripts in IDE into Java, C#, Perl, Php, Python, Ruby Scripts. We can do most of the testing in selenium with some effort. Selenium can be deployed on windows, Linux, and Mac. We can do functional testing, browser compatibility testing.