Not all instructors implement active-learning strategies in a way that maximizes student outcomes. One potential explanation for variation in active-learning effectiveness is variation in the teaching knowledge an instructor draws upon. Guided by theoretical frameworks of pedagogical content knowledge and pedagogical knowledge, this study investigated the teaching knowledge instructors used in planning, implementing, and reflecting on active-learning lessons in large courses. We used a preinstruction interview, video footage of a target class session, and a postinstruction interview with stimulated recall to elicit the teaching knowledge participants used. We then conducted qualitative content analysis to describe and contrast teaching knowledge employed by instructors implementing active learning that required students to generate their own understandings (i.e., generative instruction) and active learning largely focused on activity and recall (i.e., active instruction). Participants engaging in generative instruction exhibited teaching knowledge distinct from that of participants focused on activity. Those using generative instruction drew on pedagogical knowledge to design lessons focused on students generating reasoning; integrated pedagogical content knowledge and pedagogical knowledge to plan lessons to target student difficulties; and created opportunities to develop new pedagogical content knowledge while teaching. This work generated hypotheses about the teaching knowledge necessary for effective, generative active-learning instruction.