This paper develops the founding elements of the concept of Communities of Practice (CoPs) by elaborating on the learning processes happening at the heart of such communities. In particular, it provides a consistent perspective on the notions of knowledge and of knowledge sharing that is compatible with the ‘DNA’ of this concept, i.e. learning entailing an investment of identity and a social formation of a person. It does so by drawing richly from the work of Michael Polanyi and his conception of Personal Knowledge, and thereby it clarifies the scope of CoP, it ‘brings knowledge back’ into CoPs as a technical term, and it offers a number of new insights into how to make such social structures ‘work’ in professional settings. The conceptual discussion is substantiated by the findings of a qualitative empirical study in the National Health Service (NHS) Scotland. As a result, the process of ‘thinking together’ is conceptualized as people mutually guiding each other through their understandings of the same problems in their area of mutual interest, and this way indirectly sharing tacit knowledge. This collaborative learning process, it is argued, is what specifically brings CoPs to life and not the other way round.