This study of the nature of pedagogical expertise examines the thinking and actions of a small number of expert and novice science teachers. We draw upon a conceptual framework that characterizes teaching as a complex cognitive skill determined in part by the nature of a teacher’s knowledge system to explain patterns in participants’ planning, teaching and post-lesson reflections. The chapter begins with a discussion of the conceptual framework and a description of the investigation and its findings. We then present abbreviated case descriptions of four teachers to illustrate patterns in the expert and novice teachers’ thinking and actions. These patterns are examined in terms of the conceptual framework of teaching as a complex cognitive skill. Finally, we compare this investigation with a similar study of mathematics instruction and offer several suggestions for future research.