Software development and acquisition require knowledge and experience in many areas of software engineering. Experience helps people to make decisions under uncertainty, and to find better compromises. Experience-based process improvement considers experience as a prerequisite for competent behavior in software development. There is usually a repository to store experiences and to make it available for reuse. At DaimlerChrysler, we have been building those repositories for more than five years. We learned to concentrate on certain properties that seem to be key success factors for experience repositories. During our experience-based work in business units, five key quality aspects have been identified that determine the chances for success of an experience repository. The quality criteria can be used to analyze a given repository; or they can be applied to guide the construction of more effective experience repositories.