Welcome to the 5th International Conference on Open Source Systems! It is quite an achievement to reach the five-year mark - that's the sign of a successful enterprise. This annual conference is now being recognized as the primary event for the open source research community, attracting not only high-quality papers, but also building a community around a technical program, a collection of workshops, and (starting this year) a Doctoral Consortium. Reaching this milestone reflects the efforts of many people, including the conference founders, as well as the organizers and participants in the previous conferences. My task has been easy, and has been greatly aided by the hard work of Kevin Crowston and Cornelia Boldyreff, the Program Committee, as well as the Organizing Team led by Bjoern Lundell. All of us are also grateful to our attendees, especially in the difficult economic climate of 2009. We hope the participants found the conference valuable both for its technical content and for its personal networking opportunities. To me, it is interesting to look back over the past five years, not just at this conference, but at the development and acceptance of open source software. Since 2004, the business and commercial side of open source has grown enormously. At that time, there were only a handful of open source businesses, led by RedHat and its Linux distribution. Companies such as MySQL and JBoss were still quite small.