This paper addresses the question of the role of ostracism in democratic Athens. I argue that the frequent expulsion of aristocrats by rival aristocrats in the predemocratic polis is the key to understanding the function of ostracism in the democratic polis. I show that aristocratic "politics of exile" was a fundamental political problem in the archaic polis and that democratic political power, symbolized by the institution of ostracism, was the polis' solution to the problem. In the archaic polis, the expulsion of aristocrats often led to an endless cycle of exiles and returns and consequent political instability. In 508/7, the Athenian demos usurped the power over decisions of exile by banishing Isagoras and recalling Cleisthenes from exile. By taking control of decisions of exile, the Athenian demos took over political power insofar as power over decisions of exile had become synonymous with political power. The creation of the institution of ostracism, whereby the people decided collectively whether to banish a single citizen for ten years, provided not only a mechanism for the symbolic expression of democratic power, but also a means for the practical and ideological distinction between oligarchic and democratic rule. While aristocrats had banished their opponents without limit in numbers and duration of exile, the Athenian democracy decided each year whether to banish a single individual for a limited time period. In most years, the Athenian demos chose not to exercise the power of exile. Nevertheless, the annual question in the assembly served to remind aristocrats that the demos held the fundamental political power, and hence deterred aristocrats from attempting to overthrow the democracy. The limited nature of ostracism further deterred exiled aristocrats from attempting to return by force. Finally, the infrequent and moderate use of exile as a means of resolving political conflict helps to explain the extraordinary stability of the Athenian democracy.