Nudging interventions are broadly defined as a rearrangement of a choice context that gently suggests a specific choice. Their increasing popularity has attracted attention and discussion from researchers, policy makers, and practitioners alike. After some applications to domains such as health, environmental issues, and retirement savings, the next step in nudging is to understand the psychological boundary conditions when applied to varied domains of daily life. It is yet unclear for example to what extent nudging interventions can be transparent without losing effectiveness, or how permanent the effects are. These unresolved questions may have contributed to heated political and scientific discussions, on for example the ethics of using nudges in the public health domain, due to the missing scientific evidence. Indeed, this popularity may have led to forget how nudging harnesses insights from decades of research in psychology to change people's behavior. The aim of this paper is to shift the focus back to the psychological premises nudges were built upon. It summarizes several outstanding questions that future research in the psychology of nudging should address. Advancing research in nudging will help to improve our understanding of applied nudging interventions and provide clarity to debates such as ethical appropriateness, effectiveness, and public approval.