In order to identify the components of knowledge that effective clinical teachers of medicine need, the author carried out a qualitative study of six distinguished clinical teachers in general internal medicine in 1991. Using data from interviews, a structured task, and observations of each ward team, he identified six domains of knowledge essential to teaching excellence in the context of teaching rounds: clinical knowledge of medicine, patients, and the context of practice, as well as educational knowledge of learners, general principles of teaching and case-based teaching scripts. When combined, these domains of knowledge allow attending physicians to engage in clinical instructional reasoning and to target their teaching to the specific needs of their learners. The results of this investigation are discussed in relation to both prior research on teacher knowledge, reasoning, and action and faculty development in medicine.