What was the Holy Roman Empire in the fourteenth to sixteenth centuries? At the turning point between the medieval and early modern periods, this vast central European polity was the continent’s most politically fragmented. The imperial monarchs were often weak and distant, while an array of regional actors played autonomous political roles. The Empire’s obvious differences from more centralized European kingdoms have stimulated negative judgements and fraught debates, expressed in the historiographical concepts of fractured ‘territorial states’ and a disjointed ‘imperial constitution’. This book challenges these interpretations through a wide-ranging case study of Upper Germany between 1346 and 1521. By examining the interactions of princes, prelates, nobles, and towns comparatively, it demonstrates that a range of actors and authorities shared the same toolkit of rituals, judicial systems, and configurations of government. Crucially, Upper German elites all participated in leagues, alliances, and other treaty-based associations. As frameworks for collective activity, associations were a vital means of enabling and regulating warfare, justice and arbitration, and even lordship and administration. The prevalence of associations encouraged a mentality of ‘horizontal’ membership of political communities, so that even the Empire itself came to be understood and articulated as an extensive and multi-layered association. On the basis of this evidence, the book offers a new and more coherent vision of the Holy Roman Empire as a sprawling community of interdependent elites who interacted within the framework of a shared ‘associative political culture’, which constituted an alternative structure and pathway of political development in pre-modern Europe.