By adopting the concept of divided power, this book makes a case for studying ancient Greek political decision-making as a multi-layered system of delegation and legal control. Scholars have debated on the nature and locus of sovereignty in the Classical and Hellenistic Greek poleis by adopting institutional, rhetorical or ideological approaches. By concentrating on the institutional design of decree-making, this new study aims to move beyond unitary and hierarchical understandings of sovereignty and presents a new view of power as divided and horizontally-organized between different decision-making institutions each one with its discourse and expertise. This book aims to reorient the interpretation of Greek political decision-making through a new institutionalist perspective, reaffirming the normative importance of political institutions as factors shaping collective the behaviour of decision-makers. Part I explores the mechanisms of delegation of deliberative power in decree-making in Classical Athens, Mytilene and Hellenistic Megalopolis. Part II discusses the procedures of legal control and judicial review in the Classical and Hellenistic periods. Divided Power was a feature of both democratic and non-democratic constitutions across the Greek World and the analysis of its institutional manifestations contributes to our understanding of discourses and norms of political life in the ancient Greek city-states.