Welcome to the 5th International Conference on Open Source Systems! 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.

Keynote Speakers.- Open Source Is Changing the Way Work Gets Done.- How Open Source Can Still Save the World.- Papers.- Domain Drivers in the Modularization of FLOSS Systems.- Design Evolution of an Open Source Project Using an Improved Modularity Metric.- Software Engineering in Practice: Design and Architectures of FLOSS Systems.- Beyond the Business Model: Incentives for Organizations to Publish Software Source Code.- Opening Industrial Software: Planting an Onion.- Providing Commercial Open Source Software: Lessons Learned.- Analysis of Open Source Software Development Iterations by Means of Burst Detection Techniques.- Heartbeat: Measuring Active User Base and Potential User Interest in FLOSS Projects.- Estimating Commit Sizes Efficiently.- The Importance of External Support in the Adoption of Open Source Server Software.- Customization of Open Source Software in Companies.- Choosing Open Source ERP Systems: What Reasons Are There For Doing So?.- Reporting Empirical Research in Open Source Software: The State of Practice.- What Does It Take to Develop a Million Lines of Open Source Code?.- An Empirical Study of the Reuse of Software Licensed under the GNU General Public License.- Quality of Open Source Software: The QualiPSo Trustworthiness Model.- Challenges of the Open Source Component Marketplace in the Industry.- A Survey on Firms’ Participation in Open Source Community Projects.- FLOSS UX Design: An Analysis of User Experience Design in Firefox and OpenOffice.org.- Integrating HCI Specialists into Open Source Software Development Projects.- A Survey of Usability Practices in Free/Libre/Open Source Software.- Reassessing Brooks’ Law for the Free Software Community.- “Peeling the Onion”.- Group Maintenance Behaviors of Core and Peripherial Members ofFree/Libre Open Source Software Teams.- What Constitutes Open Source? A Study of the Vista Electronic Medical Record Software.- Openness to Standard Document Formats in Swedish Public Sector Organisations.- Using FLOSS Project Metadata in the Undergraduate Classroom.- Undergraduate Research Opportunities in OSS.- Workshops.- 4th International Workshop on Public Data about Software Development.- First International Workshop on Building Sustainable Open Source Communities (OSCOMM 2009).- 1st International Workshop on: ‘Designing for Participatory Learning’ Building from Open Source Success to Develop Free Ways to Share and Learn.- A Joint Workshop of QACOS and OSSPL.- NESSI OSS Workshop.- Workshop – Serious Games and Open Source: Practice and Futures.- Posters.- Assurance Evaluation for OSS Adoption in a Telco Context.- Assessing FLOSS Communities: An Experience Report from the QualOSS Project.- Release Mismanagement in Open Source.- Libre Software in Spanish Public Administrations.- The Case Study of an F/OSS Virtualization Platform Deployment and Quantitative Results.- Panels.- Panel: Open Source in the Public Sector.- Panel: Governance in Open Source Projects and Communities.

This book constitutes the refereed proceedings of the 5th IFIP WG 2.13 International Conference on Open Source Systems, OSS 2009,  held in Skövde, Sweden, in June 2009. The 28 revised full papers presented together with the abstracts of two keynote speeches, 6 workshop introductions, 5 posters and 2 panels were carefully reviewed and selected from 59 submissions. The papers reflect the international communities of active OSS researchers from a number of academic fields ranging from computer science and information science to economics and business studies. They feature topics extending from architectures of OSS, mining OSS data, empirical research on OSS, user involvement in OSS design and OSS communities to commercial OSS, company participation in OSS, and public sector and educational usage of OSS.