The part played by tacit skills and knowledge in work performance is well recognised but not well understood. These implicit or hidden dimensions of knowledge and skill are key elements of "mastery," which experienced workers draw upon in everyday activities and continuously expand in tackling new or unexpected situations. This paper, based on the ESRC Teaching and Learning Research Network on Workplace Learning, argues that it is important to understand better how tacit forms of key competences can contribute to sustaining learning outcomes in different types of learning environments. (Contains 2 figures.)