This book constitutes the refereed proceedings of the 5th Biannual German Conference on Knowledge-Based Systems, XPS-99, held in W?rzburg, Germany in March 1999. The book presents four invited surveys by leading scientists together with 14 revised full papers selected from more than 50 papers presented at the meeting. The papers address research issues in knowledge engineering, knowledge-based diagnosis, knowledge-based configuration, and case-based reasoning as well as advanced applications of knowledge-based systems in various fields.