A special year like 1999 invites one to draw a balance of what has been achieved in the roughly 30 years of research and development in knowledge based systems (still abbreviated as XPS following the older term "expert systems") and to take a look at th what the future may hold. For the 5 German conference on knowledge-based systems we therefore asked current and former speakers of the four working groups (FG's) in the subdivision of knowledge-based systems (FA 1.5) of the German association of Informatics (GI) to present a survey of and future prospects for their respective fields: knowledge engineering, diagnosis, configuration, and case-based reasoning. An additional 14 technical papers deal with current topics in knowledge-based systems with an equal emphasis on methods and applications. They are selected from more than 50 papers accepted in the 4 parallel workshops of XPS-99: a) Knowledge Management, Organizational Memory and Reuse, b) various fields of applications, c) the traditional PuK Workshop (planning and configuration), and d) the GWCBR (German workshop on case-based reasoning). The other papers presented at these workshops are not included in this volume but are available as internal reports of Würzburg university together with the exhibition guide that emphasizing tool support for building knowledge based systems.