Propositions about the nature of expertise, in general, and expertise in pedagogy, in particular, are discussed. The time needed to develop expertise in teaching and the highly contextual nature of teachers' knowledge are also discussed. Four theories of teacher development are presented, with an elaboration on the heuristic value of the theory of Dreyfus and Dreyfus (1986). Examples from the pedagogical literature are used to illustrate this theory. The recent research establishing causal relationships between those identified as experts in teaching and their students' academic achievement is also presented. This research allows those who study expertise in teaching to have a more objective measure for identifying and studying expert pedagogues.