This chapter starts from a critical engagement with the claim that people have a moral obligation to participate in medical research because they stand to benefit from the medical knowledge it produces. It argues that this claim requires one to presume that medical knowledge serves all, without restrictions. It introduces the idea of medical tissue and data collections as infrastructures for the purpose of population-based medical research as a heuristic to interrogate this assumption and to explore value commitments in contemporary medical research. To develop the heuristic further, the chapter positions it in relation to discourses of promise and hope, transforming research practice and governance, and questions of ethics and justice in the context of the supposedly growing scale of contemporary medical research. The chapter then introduces the analysis of four case studies of tissue and data collections for medical research discussed in this book.