Communities of practice (CoPs) have been identified as a means to enable acquisition and sharing of tacit knowledge in organizations. CoPs complement or even replace approaches where the focus is on storing and retrieving codified knowledge in documents and systems. This paper highlights some of the problematic aspects of using CoPs as a tool to improve knowledge sharing. Theories on coordination are put forward as a theoretical frame to explain the role of CoPs in organizational functioning. Our study of CoPs in the Amsterdam Police Force shows how CoPs in organizations face several dilemmas. How can CoPs be self-directing and simultaneously contribute to organizational performance? How can they utilize the IT opportunities of contacting anybody, any time, any place and simultaneously circumvent the threats of low commitment and poor mutual understanding in virtual groups? How can members of CoPs produce shared repertoire without falling in the trap of groupthink? How can CoPs act as environments for sharing tacit as well as explicit knowledge? Coordination theory suggests how members of CoPs in organizations can deal with such competing values by making situational choices that reflect paradoxical guidelines. We discuss how police officers use such paradoxical guidelines in organizational prototyping sessions.