This chapter explores the nature of the Athenian Empire (or Delian League) during the Peloponnesian War, discussing its administration, its finances (particularly the tribute and the tax on trade that replaced it), and the changes in its scale and ideology during this period. The final part of the chapter outlines other aspects of interstate conduct, especially attitudes to treaties and diplomacy. The non-Thucydidean evidence, both literary and (especially) epigraphic, for all of these themes is discussed, partly because Thucydides is not always the most detailed (or objective) source for these questions, and partly to draw attention to distinctive features of Thucydides’ analysis.