This PhD dissertation draws from philosophical and psychological arguments as well as from current discussions in knowledge management to put forward a conception of sales people’s customer knowledge as tacit, situational, pragmatic and action-related. Depth-interviews and observational methods are used to examine the nature of sales people’s customer knowledge; data are analysed following the grounded theory method. The empirical investigation establishes that a sales person’s knowledge of the client cannot be dissected from the relationship in which it is created and put to use. The sales person knows her customers in and through the relationship she builds and maintains with them; customer knowledge is first and foremost relational knowledge. Such relational knowledge is social in nature and it is almost exclusively experiential knowledge: it is built up, developed and changed during the interaction with the client. This view of customer knowledge sheds a new light on such issues as the use of sales automation tools, sales team interaction and sales training; most crucially, it broadens the existing cognitive selling paradigm to take account of the social construction of the sales encounter.