This book presents ideas, evidence and guidance for those interested in using the most recent advances in knowledge about learning and human development to enhance medical education's ability to form competent, caring and publicly responsible physicians. It does this by establishing the development of a professional identity in medical students and residents as a primary goal of medical education. This new approach is emerging from experience and experiment by medical educators articulating a new way of understanding their mission. It is an optimistic book - the voices are those of the leaders, theorists and experienced practitioners who have found in this new approach a promising way to confront the challenges of a new era in medicine. It summarizes the theoretical basis of identity formation, outlines our current knowledge of how best to assist learners as they acquire a professional identity, and addresses the issue of assessment of progress towards this goal.