In acquiring a skill by means of instruction and experience, the student normally passes through five developmental stages which we designate novice, competence, proficiency, expertise and mastery. We argue, based on analysis of careful descriptions of skill acquisition, that as the student becomes skilled, he depends less on abstract principles and more on concrete experience. We systematize and illustrate the progressive changes in a performer's ways of seeing his task environment. We conclude that any skill training procedure must be based on some model of skill acquisition, so that it can address, at each stage of training, the appropriate issues involved in facilitating advancement.