Open source software development is the process by which open source software (or similar software whose source is publicly available) is developed. These are software products “available with its source code and under an open source license to study, change, and improve its design”. Examples of popular Open source software product are Mozilla Firefox and the OpenOffice.org Suite. The Open source software development method is very unstructured, because no clear development tools, phases, etc. have been defined like with development methods such as DSDM. Instead, every project has its own phases. There are, however, generalities between Open source development projects. In this entry these generalities will be described.