For more than a century, the medical profession has made enormous efforts to understand and treat women’s reproductive bodies. But only recently have researchers begun to ask basic questions about how men’s health matters for reproductive outcomes, from miscarriage to childhood illness. What explains this gap in knowledge, and what are its consequences? Rene Almeling examines the production, circulation, and reception of biomedical knowledge about men’s reproductive health. From a failed nineteenth-century effort to launch a medical specialty called andrology to the contemporary science of paternal effects, there has been a lack of attention to the importance of men’s age, health, and exposures. Analyzing historical documents, media messages, and qualitative interviews, GUYnecology demonstrates how this non-knowledge shapes reproductive politics today.