What should happen to a concept as it loses real-world application? The concept of ‘middle power’ rose to prominence in the mid-20th century, establishing an influential practitioner–scholarly nexus over the next several decades. This prestigious history came at a cost, embedding three core assumptions into the concept: that middle powers are International in focus, Multilateral in method, and Good Citizens in conduct. While there have been significant attempts by scholars to reform the concept, middle power theory has proven inseparable from these assumptions. In this paper, we examine six middle power states (Canada, Australia, South Korea, Indonesia, Turkey, and Mexico) and show middle power theory no longer helps us distinguish or interpret these states. Changes in the international environment suggest this finding will endure. As such, we argue for the historicization of the concept of ‘middle power’. We conclude by identifying a series of analytical puzzles which researchers will need to address to develop an appropriate conceptual lexicon for theorizing this type of state in the 21st century.