For a long time the importance of requirements has been underestimated. In many software engineering methods they were just the raw material that was used to build analysis and design models. The emphasis was on these models, not the requirements. Over the last few years requirements engineering has become a serious discipline with its own body of knowledge, its own conferences and publications. This paper discusses requirements in the context of systems. It classifies and categorizes them and defines processes to bring them to a level that is useable in modern system development methods.