This paper reviews five broad types of research that are designed to determine whether or how teacher education has made a difference to teachers. The five genres consist of: (1) a line of reasoning about where one might look for the effects of teacher education and how one might design a research study to see the impact of teacher education; (2) comparison studies in which teachers who have received formal teaching credentials are contrasted with teachers who have not; (3) studies in which researchers ask teachers what they have learned from teacher-education programs; (4) experimental studies in which different approaches to teacher education are compared; and (5) longitudinal case studies in which teachers are followed over time to see how their thinking changes as they participate in teacher education. In this review, three questions are asked of each genre: What aspects of teacher education does it examine? What outcomes does it look for? and What kind of reasoning does it use to develop a link between these aspects of teacher education and these outcomes? Suggestions are offered for researchers on ways to strengthen their research designs. (Author/LL)