Planet Mercury is both difficult to observe and difficult to reach by spacecraft. Just one spacecraft, Mariner 10, flew by the planet 30 years ago. An upcoming NASA mission, MESSENGER, will be launched this year and will go into orbit around Mercury at the end of this decade. A European mission is planned for the following decade. It's worth going there because Mercury is a strange body and the history of planetary exploration has taught us that strangeness gives us insight into planetary origin and evolution.Mercury is intrinsically the densest of the planets and it must be iron‐rich. Perhaps not coincidentally it has a significant magnetic field, which may mean that it can generate a field, much as Earth generates a field. Yet Mercury is small; it has an old, heavily cratered surface, and very little atmosphere. It also has unusual spectroscopic and radar characteristics. We understand it far less well than any other planet inward of Uranus.