Nudges are soft interventions designed to influence human behaviour. They are increasingly used by policymakers around the world and generate heated controversies over their ethical acceptability. In this article, I depict nudges as science-based technologies, point to new ethical responsibilities they raise—when they are used but also when they are not used—and to the necessity of establishing clear evaluation criteria. I then elaborate a four-level framework for evaluating the acceptability of particular nudges. The second part of the article focuses specifically on 'shared preference justifications' (SPJ), that is, arguments of the sort 'nudges help people make better choices as judged by themselves'. I explain why constructing SPJs is important but difficult and propose a framework for doing so. This framework takes autonomy concerns seriously without overstating them and provides a conceptual basis for the development of nudge evaluation procedures easily applicable by decision-makers.