Book excerpt: Information systems are large repositories of factual and inferential knowledge intended to be queried and maintained by a wide variety of users with different backgrounds and work tasks. The community of potential information system users is growing rapidly with advances in hardware and software technology that permit computer/communications support for more and more application areas. Unfortunately, it is often felt that progress in user interface technology has not quite matched that of other areas. Technical solutions such as computer graphics, natural language processing, or man-machine-man communications in office systems are not enough by themselves. They should be complemented by system features that ensure cooperative behavior of the interfaces, thus reducing the training and usage effort required for successful interaction. In analogy to a human dialog partner, we call an interface cooperative if it does not just accept user requests passively or answer them literally, but actively attempts to understand the users' intentions and to help them solve their applica tion problems. This leads to the central question addressed by this book: What makes an information systems interface cooperative, and how do we provide capabilities leading to cooperative interfaces? Many answers are possible. A first aspect concerns the formulation and accep tance of user requests. Many researchers assume that such requests should be formulated in natural language.