In this lecture, I present an initial theoretical approach for investigating mechanisms by which medical knowledge is lost.  Drawing on theories in the sociology of knowledge and STS, I will describe these mechanisms, in a preliminary way. “Forgetting” has mostly been seen as the province of psychology, seeking to understand how individuals shed knowledge. Collectives are more than the sum of their parts, of course, and the processes by which institutions, disciplines, and fields of practice forget demand their own attention. To understand how the skills required for vaginal breech delivery were forgotten, I track changes in guidelines, curriculums, hospital policies, scholarly articles, medical school syllabi, continuing medical education, and much more. These macro-factors diminished the clinical practice of vaginal breech delivery, reframed it as a taboo, and more. Investigating the mechanics of collective forgetting in medicine will increase our understanding of how medicine progresses and will make a case for the study of collective forgetting in other institutional contexts, as well.