Many modern knowledge intensive organisations rely on knowledge sharing communities, often called ‘communities of practice. These communities can be found in many organisations, but their forms and functions appear to be quite diverse. This implies that questions concerning the functioning of communities, (how do they work) and questions concerning success conditions (how to organize and facilitate them) cannot be answered in a general way. The purpose of this article is to develop the theory in this area by discovering basic types of knowledge communities, underlying the diversity of knowledge sharing groups. Through an analysis of the literature and of a series of communities in large organisations, three basic dimensions, i.e. institutionalisation, connectivity and virtuality, and five basic types of knowledge communities are identified, i.e. strategic communities, informal communities, knowledge networks, interest groups and Delphi groups.