This chapter examines research on professional development, or PD, focusing specifically on underlying assumptions about the nature of teaching and the nature of teacher learning. It examines PD programs according to their assumptions about what teachers need to learn, and it examines PD studies according to how and when they expect to see evidence of teacher learning. The chapter seeks to provide a broad view of how we think about teaching and teacher learning and to examine our underlying assumptions both about teaching and about how PD is expected to improve teaching. With respect to program effectiveness, the chapter raises questions about the extent to which effective PD programs can be replicated; with respect to our study designs, it raises questions about how teacher learning occurs and when and how we should expect to see program effects on teachers' practices. The chapter also offers some suggestions for future research design.