This article 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 article 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 data drawn on in this article consist of case studies, spanning 18 months, of four ESL teachers in Hong Kong.