To acquire expertise it is necessary to have social contact with those who already have it. Only this way can the tacit knowledge associated with a domain of expertise be transferred. A 'Periodic Table of Expertises' (from the book Rethinking Expertise) will be presented while the talk will concentrate on the difference between 'Interactional Expertise' and 'Contributory Expertise'. The former is acquired via prolonged immersion in the linguistic discourse of a domain without physical practice. Recently it has been argued that language is the dominant means of learning any human expertise, including practical expertises. This will be explained. The overall programme of research associated with these ideas will be outlined.