This paper attempts to identify the distinctive qualities of successful veteran teachers, referred to as "expert teachers", which separates them not only from novice teachers but more importantly from experienced non-expert teachers. Based on earlier case studies, this paper maintains that the critical differences between expert and non-expert teachers are manifested in three dimensions: their ability to integrate aspects of teacher knowledge in relation to the teaching act; their response to their contexts of work, and their ability to engage in reflection and conscious deliberation. The paper further addresses the question of why some teachers become experts while others remain experienced non-experts by examining the developmental processes of the experienced teachers in the case studies. The findings suggest that engagement in exploration and experimentation in teaching and learning, in problematizing the unproblematic, and in tasks which challenge teachers to extend their competence are crucial to the development of expertise. The implications for teacher development are discussed. (Contains 1 table and 2 notes.)