This double volume presents the state of the art in research on the microfoundations of institutions. In this introductory chapter, we develop an overview of where the emerging microfoundational agenda in institutional theory stands and in which direction it is moving. We discuss the questions of what microfoundations of institutions are, what the “micro” in microfoundations represents, why we use the plural form (microfoundations vs microfoundation), why microfoundations of institutions are needed, and how microfoundations can be studied. Specifically, we highlight that there are several traditions of microfoundational research, and we outline a cognitive, a communicative and a behavioral perspective. In addition, we explain that scholars tend to think of microfoundations in terms of an agency, levels, or mechanisms argument. We delineate key challenges and opportunities for future research and explain why we believe that the debate on microfoundations will become a defining element in the further development of institutional theory.