This chapter develops the notion that Frankfurt School research both inside and outside of IR is in a state of crisis. First, I propose a general definition of Frankfurt School Critical Theory that specifies its basic characteristics and establishes the standards by which its condition can be evaluated. Second, I introduce the subject area that constitutes the primary literature interrogated in this book: Frankfurt School research in International Relations. I discuss two sub-literatures of Critical Theory that have, from the 1980s onwards, engaged in the study and critique of international politics—and both of which are today in a state of stagnation. These are the literature on Cosmopolitan Democracy and the project of Critical International Relations Theory. Lastly, I relate the current predicament of Critical Theory in IR to broader discussions over the state of Frankfurt School theory that are taking place in political theory, sociology and philosophy. Bringing together these separate conversations, I argue that the contemporary impasse of Critical Theory of IR should be understood as a particular instance of the generalised crisis of critique of Frankfurt School theorising.