In this chapter, the author examines the ways in which anthropologists, sociologists, and science and technology studies scholars working in different parts of the world have ethnographically engaged with sperm as a substance. Carried out in different parts of the world, ethnographic research has shown how sperm as a substance indexes masculinity, vitality, and “strength,” both individually and collectively; has become commodified and exchangeable through the work of sperm banks; has led to complex negotiations of kinship; and acts as a sentinel of today's global environmental crisis. The making of technosemen has been undergirded by what can be thought of as technologies of assurance – a configuration of strategies and techniques within which certain persons, activities, objects and/or substances come to be vouched for over others.