Open source communities are groups of sometimes hundreds if not thousands of individuals with different interests, backgrounds and motives. Many participants are volunteers, who are not paid to take part in the communities. Furthermore, many never get to meet each other in real life. They meet virtually, on the Internet. Yet they are able to collectively develop highly complex software that has proven to work and that is viewed as a viable alternative in todays software market. The question is how? This book provides a detailed account of the organization of open source communities like Linux and Apache. Questions are answered like: How are the development efforts of hundreds of people coordinated? How are conflicts between programmers resolved? How do the communities decide whether to include a piece of source code in a software program? One of the important findings is that the organization of open source communities cannot be understood based on collective institutions. Rather, the organization of open source communities can be understood based on the behavior of the individual participants in the communities. Further, this individual behavior can be described with a limited number of behavioral rules.