The objective of this thesis is to develop a theoretical understanding of team-based knowledge sharing and creation in professional cyber-communities. In order to provide contextual richness in environmental, information technology, project, organizational, group, and individual contexts, we apply the grounded theory approach to discover essential factors affecting team-based knowledge sharing and creation across organizational boundaries. Moreover, the thesis summarizes five types of processes of team-based knowledge sharing and creation in professional cyber-communities, defined by the sequence of causal conditions, action/interactional strategies, and consequences, and concludes nine main propositions that can be tested empirically in the future research. The research results will support sustained development of professional cyber-communities and be a basis for further studies of team-based knowledge sharing and creation across organizational boundaries.