The planet Mercury is sufficiently close to the Sun to pose a major challenge to spacecraft exploration. The Mariner 10 spacecraft flew by Mercury three times in 1974–1975 but viewed less than half of the surface. With the three flybys of Mercury by the MESSENGER spacecraft in 2008–2009 and the insertion of that probe into orbit about Mercury in 2011, our understanding of the innermost planet substantially improved. In its four years of orbital operations, MESSENGER revealed a world more geologically complex and compositionally distinctive, with a more dynamic magnetosphere and more diverse exosphere–surface interactions, than expected. With the launch of the BepiColombo dual-orbiter mission, the scientific understanding of the innermost planet has moved another major step forward.