Monetary incentives are increasingly used to help motivate survey participation. This article summarizes several theories underlying the use of incentives and briefly reviews research demonstrating their intended and unintended effects on response rates, sample composition, response bias, and response quality. It also considers the evidence for the effectiveness of incentives in reducing nonresponse bias. Institutional review boards have begun to ask whether, and under what conditions, the use of monetary incentives to induce participation might be coercive and to question the use of such incentives in surveys of "vulnerable" populations, including surveys of injury and violence. The article reviews the ethical principles underlying the requirement for voluntary informed consent as well as current regulations and a broad theoretical and empirical literature bearing on this question, concluding that incentives are never coercive. The question of whether they exert "undue influence" in a specific situation is more difficult, but it may be the wrong question to ask. The article concludes with several recommendations designed to ensure the ethical use of incentives in surveys on violence and injury.