The notion of the “expert patient” has become quite prominent in recent years. In many countries, patients are invited to sit on various committees with biomedical specialists to share their expertise on diseases and health problems with various institutions. This situation stands in stark contrast with the medical paternalism that prevailed not so long ago. How did this come to pass? What is the nature of the patient expertise that is being solicited? How have health policies changed as a result? These are the questions that we examine in this article. In light of previous and recent fieldwork that we have done, and a selection of case studies reported in the literature, our objective is to revisit a constellation of notions such as “expert patient,” “lay expert,” “lay expertise,” “experiential knowledge,” and “expert of experience” that are not only analytical tools for STS scholars and social scientists, but also part and parcel of today’s institutional parlance. This article is divided into four sections. In the first section, we focus on the production of experiential knowledge by patient associations. In the second section, we show how they also strive to transform the content of credentialed expertise in order to better align this expertise with their concerns. The third section illustrates “lay expertise in action” and addresses the following questions: What changes do patient associations bring to health policymaking? How do they achieve these changes? In the fourth and last section, we examine the institutionalization of the “expert patient,” which manifests not only in legislation that mandates patient participation, but also in a variety of initiatives implemented by institutions to mobilize patient participation in medical research and health programs. In the conclusion, we suggest a few avenues for future research that may complement the corpus of knowledge on lay expertise on aspects that thus far remain less investigated.