How does "self-governance" happen in Wikipedia? Through in-depth interviews with 20 individuals who have held a variety of responsibilities in the English-language Wikipedia, we obtained rich descriptions of how various forces produce and regulate social structures on the site. Although Wikipedia is sometimes portrayed as lacking oversight, our analysis describes Wikipedia as an organization with highly refined policies, norms, and a technological architecture that supports organizational ideals of consensus building and discussion. We describe how governance on the site is becoming increasingly decentralized as the community grows and how this is predicted by theories of commons-based governance developed in offline contexts. We also briefly examine local governance structures called WikiProjects through the example of WikiProject Military History, one of the oldest and most prolific projects on the site.