What exactly is a nudge, and how do nudges differ from alternative ways of modifying people's behavior, such as fines or penalties (e.g. taxing smokers) and increasing access to information (e.g. calorie counts on restaurant menus)? We open Section 2 by defining the concept of a nudge and move on to present some examples of nudges. Though there is certainly a clear concept of what a nudge is, there is some confusion when people design and talk about nudges in practice. In Sections 3 and 4, then, we discuss policies and technologies that get called nudges mistakenly as well as borderline cases where it is unclear whether people are being nudged. Understanding mistaken nudges and borderline cases allows citizens to consider critically whether they should support “alleged” nudge policies proposed by governments, corporations, and non-profit organizations. There are also important concerns about the ethics of nudging people's behavior. In Section 5 we review some major ethical and political issues surrounding nudges, covering both public anxieties and more formal scholarly criticisms. If nudges are to be justified as an acceptable form of behavior modification in democratic societies, nudge advocates must have reasons that allay anxieties and ethical concerns. However, in Section 6, we argue that nudge advocates must confront a particularly challenging problem. A strong justification of nudging, especially for pluralistic democracies, must show that nudge designers really understand how different people re-interpret the meaning of situations after a nudge has been introduced into the situations. We call this the problem of “semantic variance.” This problem, along with the ethical issues we discussed, makes us question whether nudges are truly viable mechanisms for improving people's lives and societies. Perhaps excitement over their potential of nudges is exaggerated.