The open source model holds great promise for computational science and engineering (CS&E) software. Benefits can include heightened technical quality, a larger and more diverse developer base, integration and testing in myriad environments, and potential for flexible component reuse in new applications. Open source CS&E software fosters open, reproducible research and supports an open science process. Yet achieving truly collaborative and open software can be challenging: How does one create a vibrant, engaged community? How should contributions and potential conflicts be managed? What are the business and communication models? Representing exemplar software projects, the speakers in this session will emphasize benefits and challenges of open source CS&E software, and advise on best practices to succeed in collaborative development.