Abstract The chapter briefly retraces the intellectual history of federalism and offers an account of federalism from the point of view of constitutional theory. In the chapter, federalism is presented as a constitutional principle that advocates division of authority and dispersion of powers among different societal groups and organizations, to reflect what Elazar dubbed ‘self-rule’ and ‘shared rule.’ It is also construed as an umbrella term that includes different experiences. In fact, while the US federation emerged in 1787 is usually regarded as the quintessential classic federal model, in recent years federal principles and elements have been adapted to a variety of situations, blending centripetal and centrifugal forces, federal and non-federal experiences, in a process leading to always novel experimentations. The discussion of cities in federal constitutional theory—which lies at the core of this volume—is a compelling illustration of this recent intellectual evolution in federalism design. The existing literature on federalism and federal theory is truly gigantic and therefore it is impossible to treat it exhaustively in a very limited space. The present chapter will thus identify what the author believes are the intellectual milestones of federal history and theory (section II) before delving into the historical treatment (or lack thereof) of cities as federal constitutional entities or ‘units’ (section III).