The success and the massive adoption of Web applications and services are pushing the community to increase and enhance their developments. By that way, the complexity and size of Web-based systems are definitely growing. Accordingly, the need for sophisticated and complete methods used to test the reliability and security aspects of Web systems is increasing as well. Quality and relevant test cases development can achieve up to 70% of the total cost of the project when these test cases are hand crafted. Because of this, the industry and the research community are making big efforts to automate test cases generation. That is the reason why the test generator must be supplied with a precise and unambiguous semantic description of the implementation under test (IUT), i.e. a formal model. This chapter presents two methodologies to attain automatic test cases generation: The first one applies extended finite state machines to model Web services composition described in WS-BPEL, while the other one uses UML to model Web applications. Together with the formal models of the web systems, this chapter presents methods for conformance and non-regression test generation.