This book constitutes the refereed proceedings of the 6th International Workshop on Learning Software Organizations, LSO 2004, held in Banff, Canada in June 2004. The 13 revised full papers and 3 revised short papers presented together with an introduction by the volume editors were carefully reviewed and selected for presentation. The book is devoted to technical, organizational, and social solutions to problems of learning from previous experiences and codifying the resulting best practices so that they can be used in subsequent software development efforts. The papers are organized in topical sections on experience-based information systems, software maintenance, communities of practice, planning LSOs, and case studies and experience reports.