Is paternalism legitimate? Might it increase welfare? When would it compromise autonomy in some impermissible way? The outpouring of empirical work on cognitive biases, and on departures from perfect rationality, has led to a wholesale rethinking of paternalism and its limits. Over the last decades, three camps have emerged: (1) coercive paternalists, who urge that behavioral findings undermine John Stuart Mill's Harm Principle and greatly strengthen arguments for paternalistic mandates and bans; (2) libertarian paternalists, who urge that behavioral findings justify a host of paternalistic but freedom-preserving interventions or "nudges," such as warnings, reminders, labels, and automatic enrollment; and (3) antipaternalists, who urge that behavioral findings justify only, or at most, efforts to strengthen or "boost" people's competences, or their capacities to make good choices. On welfare grounds, it is possible to identify the assumptions under which one or another approach would be best. Libertarian paternalism often has significant advantages over coercive paternalism, at least in circumstances in which choosers are heterogeneous. But when all or most choosers err, the welfarist argument for coercive paternalism is strengthened. And when choosers are not only heterogeneous but also adequately informed and free from behavioral biases, antipaternalism makes a great deal of sense.