Many industrialized countries are adjusting their social security and pension schemes in order to reduce government expenditures and to increase labor force participation rates of senior citizens. The most obvious reform in this respect is raising the standard retirement age. Yet, it has become clear that the observed individual retirement behavior cannot be explained by financial incentives alone. In this paper, Frank van Erp, Niels Vermeer, Daniel van Vuuren (all CPB) first confront the observed aggregate retirement pattern in the Netherlands with predictions of traditional economic models of retirement. They survey different explanations from psychology and sociology which may help to explain the observed retirement peaks.