Apprenticeship has a long history of success in developing skills. Despite this potential for adult education, the tacit core of apprenticeship has made it difficult to derive educational principle. This paper addresses the nature of this tacit core by intuiting generic principles from case studies of apprenticeship. Cases are taken from the Japanese tea ceremony, traditional crafts, and strategic thinking. The cases highlight a novice's cumulative growth in internal knowledge from reflectively practising a prescribed skilful process. The implications of the principles of internal knowledge and skilful process for adult education centre on instructors who are skilled experts engaged in continually improving the skilful processes they model. Implications for theory centre on an emphasis on a demand for worker participation and cooperation with management in on-the-job training.