<p>Agile software development projects rely on the diversity of team members’ expertise. This expertise, however, is not adequate on its own: it is important to leverage available expertise through expertise coordination. Expertise coordination requires team members to rely on each other for recognizing who has particular expertise, when and where they are needed, and how to access the expertise effectively. Agile teams also need to rely on outside expertise such as user experience designers, architects, and database administrators. This thesis presents a theory of expertise coordination in Agile Software Development projects. We employed semi-structured interviews, observations, and document analysis in a Grounded Theory study involving 48 Agile practitioners and external specialists. This study discovered three main categories of expertise coordination: processes of expertise coordination, strategies of managing external expertise, and management roles in supporting expertise coordination. The theory provides a new insight into how Agile teams coordinate internal and external expertise, how they utilize external specialists and outsourcers’ expertise, and how management can support expertise coordination.</p>