Self-organizing teams are a hallmark of Agile software development, directly affecting team effectiveness and project success. Agile software development, and in particular the Scrum method, emphasizes self-organizing teams but does not provide clear guidelines on how teams should become and remain self-organizing. Based on Grounded Theory research involving 58 Agile practitioners from 23 different software organizations, this thesis presents a grounded theory of self-organizing Agile teams. These teams take on spontaneous roles in order to become self-organizing. The roles are: Mentor, Co-ordinator, Translator, Champion, Promoter, and Terminator. Teams learn freedom and responsibility, cross-functionality and specialization, under iteration pressure. Influences are senior management support and level of customer involvement. This thesis will help teams and their coaches better understand their roles and responsibilities as a self-organizing Agile team.