As International Organization commemorates its seventy-fifth anniversary, the Liberal International Order (LIO) that authors in this journal have long analyzed is under challenge, perhaps as never before. The articles in this issue explore the nature of these challenges by examining how the Westphalian order and the LIO have co-constituted one another over time; how both political and economic dynamics internal to the LIO threaten its core aspects; and how external threats combine with these internal dynamics to render the LIO more fragile than ever before. This introduction begins by defining and clarifying what is “liberal,” “international,” and “orderly” about the LIO. It then discusses some central challenges to the LIO, illustrated by the contributors to this issue as well as other sources. Finally, we reflect on the analytical lessons we have learned—or should learn—as the study of the LIO, represented by scholarship in International Organization, has sometimes overlooked or marginalized dynamics that now appear central to the functioning, and dysfunction, of the order itself.