Software testing is a difficult task and testing Web-based applications may be even more difficult, due to the peculiarities of such applications. In the last years, several problems in the field of Web-based applications testing have been addressed by research work, and several methods and techniques have been defined and used to test Web-based applications effectively. This paper will present the main differences between Web-based applications and traditional ones, how these differences impact the testing of the former ones, and some relevant contributions in the field of Web application testing developed in recent years. The focus is mainly on testing the functionality of a Web-based application, even if some discussion about the testing of non-functional requirements is provided too. Some indications about future trends in Web application testing are also outlined in the paper.