When attending physicians are conducting teaching rounds, they rapidly decide what and how much to teach in response to each case presentation. How do they make these instructional decisions? The author performed a qualitative study of the instructional reasoning and actions of six distinguished clinical teachers in general internal medicine to address this question. Four data sources were used: interviews with teachers and learners, a structured task, transcripts of teaching rounds, and week-long observations of each ward team. The teachers in this study engaged in substantial amounts of planning before rounds and reflected on rounds afterwards. When listening to a case presentation during rounds, they quickly diagnosed the patient's problems and simultaneously diagnosed their learners' levels of understanding. These diagnostic assessments were used to tailor content-specific curriculum scripts for instruction. Throughout the rounds, the teachers also engaged in interactive thinking, decision making, and improvisation. The author's findings allowed him to hypothesize a model of clinical instructional reasoning and action; they contribute new insights into the interplay between reasoning in a discipline and pedagogical reasoning. Instructional reasoning and clinical reasoning were found to be closely linked through the use of scripts. The implications of these and other findings for medical faculty development are discussed.