With the advent of the Semantic Web and the technologies for its realization, the possibilities for applying ontologies as a means to define the information and knowledge semantics become more and more accepted in different domains. The nature of requirements engineering involves capturing knowledge from diverse sources, including many stakeholders with their own interests and points of view. There are, therefore, many potential uses of ontologies in Requirements Engineering (RE). The purpose of this paper is to comprehensively review and present these uses. The main contribution is the classification of approaches that include ontologies within RE, with the aim of clarifying the way in which traditional RE techniques can benefit from them. Furthermore, future trends are identified.