This paper provides a state-of-the-art review of cloud testing. Cloud computing, a new paradigm for developing and delivering computing applications and services, has gained considerable attention in recent years. Cloud computing can impact all software life cycle stages, including the area of software testing. TaaS (Testing as a Service) or cloud testing, which includes testing the cloud and testing using the cloud, is a fast developing area of research in software engineering. The paper addresses the following three areas: (1) general research in cloud testing, (2) specific cloud testing research, i.e., tools, methods, and systems under test, and (3) commercial TaaS tools and solutions