It is beyond the scope of this chapter to do justice to the various bodies of literature that have emerged in the field of (comparative) regionalism. Since the audience of this chapter is mainly students of International Relations, the focus will be on processes and structures of state-led regionalism with a focus on the delegation of policies and political authority to regional institutions. Based on this more narrow understanding of regionalism, the existing literature will be reviewed with regard to three general questions. These questions do not only require research across regions but also allow developing a common research agenda to accumulate knowledge generated about specific regions. First, what are the outcomes of regionalism? How can we describe and compare the results of the delegation of policies and political authority? Second, what are the drivers of regionalism? Why do some governments choose to delegate policies and political authority while others do not? Finally, what are the internal effects of regionalism? How does the delegation of policies and political authority impact back on the domestic structures of the states involved? Before reviewing the state of the art on these three questions, the chapter will trace the history of the study of regionalism in IR.