The skin offers exciting possibilities for human-computer interaction by enabling new types of input and feedback. We survey 42 research papers on interfaces that allow users to give input on their skin. Skin-based interfaces have developed rapidly over the past 8 years but most work consists of individual prototypes, with limited overview of possibilities or identification of research directions. The purpose of this article is to synthesize what skin input is, which technologies can sense input on the skin, and how to give feedback to the user. We discuss challenges for research in each of these areas.