Background. This paper explores the conceptual and methodological problems arising from several empirical investigations of professional education and learning in the workplace. Aims. 1. To clarify the multiple meanings accorded to terms such as ‘ non-formal learning’, ‘ implicit learning’ and ‘ tacit knowledge’, their theoretical assumptions and the range of phenomena to which they refer. 2. To discuss their implications for professional practice. Method. A largely theoretical analysis of issues and phenomena arising from empirical investigations. Analysis. The author's typology of non-formal learning distinguishes between implicit learning, reactive on-the-spot learning and deliberative learning. The significance of the last is commonly overemphasised. The problematic nature of tacit knowledge is discussed with respect to both detecting it and representing it. Three types of tacit knowledge are discussed: tacit understanding of people and situations, routinised actions and the tacit rules that underpin intuitive decision-making. They come together when professional performance involves sequences of routinised action punctuated by rapid intuitive decisions based on tacit understanding of the situation. Four types of process are involved-reading the situation, making decisions, overt activity and metacognition-and three modes of cognition-intuitive, analytic and deliberative. The balance between these modes depends on time, experience and complexity. Where rapid action dominates, periods of deliberation are needed to maintain critical control. Finally the role of both formal and informal social knowledge is discussed; and it is argued that situated learning often leads not to local conformity but to greater individual variation as people's careers take them through a series of different contexts. This abstract necessarily simplifies a more complex analysis in the paper itself.