Power hierarchies have always been present in political and philosophical discourses concerning international relations, yet agreed definitions are still missing for recurrent concepts such as that of middle power. This chapter reviews its theoretical history through an examination of four relevant historical periods: ancient times, post-classical and early modern era, 20th century and post-Cold War period. It contends that middle power terminology has evolved following the modifications of the structure of the international system through history, with Greek poleis, Chinese and Indian fiefdoms, Roman provinces, Italian city-states and middle powers in the modern sense as the polities of interest of every period. Lastly, it argues that the current lack of conceptual clarity and shared definitions therefore needs to be understood as a product of the historical determination of the middle power concept, a condition that calls for newer research in the light of the ever-changing nature of IR.