Pedagogical content knowledge has been shown to be important to the work of teachers. This paper assumes it should be incorporated into the curriculum of teacher preparation and explores when and how that might be done. Results from the literature are reported in order to raise issues needing to be resolved before this question can be answered satisfactorily. The preparation of teachers typically occurs in three stages: a liberal education together with acquisition of competence in subject matter, the professional study of education, and clinical experience in the schools. Some questions raised for discussion are: (1) to what extent does pedagogical content knowledge need to be taught at all? (2) what is the derivation of pedagogical content knowledge? (3) to what extent can teachers learn pedagogical content knowledge effectively in preservice academic settings versus inservice clinical settings? and (4) how can universities effectively teach pedagogical content knowledge? This paper neither exhausts the range of issues pertaining to when teachers should learn pedagogical content knowledge nor resolves any of them. Rather, it simply points out that this is an important area which has yet to be explored, indicates the complexity involved in trying to formulate an answer, and suggests some of the kinds of investigations that might help practitioners make better decisions regarding this aspect of teacher education. (LL)