What was a polis? The Copenhagen Polis Centre (core-funded by the Danish National Research Foundation) has recently begun a broad series of investigations into the origins, nature and development of the ancient Greek city-states (poleis). This empirical project will be grounded in a comprehensive inventory of all attested poleis of the late archaic and classical periods (ca. 600 - ca. 323 B.C.); and that in turn necessitates an attempt to establish working principles, in source-criticism and historical methodology generally, for the differentiation of poleis from communities of other types. The present volume is a collection of papers, from members of the Centre, which seek to make preliminary contributions to the clarification of such principles.