This article discusses the use of green nudges—behavioral interventions aimed at reducing negative externalities—as an environmental policy instrument. We present a new framework for classifying nudges according to how they affect behavior. Pure nudges change the choice environment to guide behavior unobtrusively. Moral nudges trigger a psychological reaction to encourage behavior change. Our review of empirical studies reveals that green nudges, pure or moral, can have a significant impact on behavior and the environment but that the effects are highly context dependent. On the basis of both our review and basic welfare economics models, we discuss key factors for policy makers to consider when choosing between implementing a green nudge and implementing conventional policy instruments.