The role of volcanism in Mercury’s geological history remains uncertain, and the dominant tectonic structures are lobate scarps interpreted as recording an extended episode of planetary contraction, issues that require global imaging to be fully examined. That Mercury has retained a global magnetic field when larger terrestrial planets have not stretches the limits of standard hydromagnetic dynamo theory and has led to proposals for a fossil field or for exotic dynamo scenarios. Hypotheses for field generation can be distinguished on the basis of the geometry of Mercury’s internal field, and the existence and size of a fluid outer core on Mercury can be ascertained from measurements of the planet’s spin axis orientation and gravity field and the amplitude of Mercury’s forced librations. The nature of Mercury’s polar deposits, suggested to consist of volatile material cold-trapped on the permanently shadowed floors of high-latitude impact craters, can be tested by remote sensing of the composition of Mercury’s surface and polar atmosphere. The extremely dynamic exosphere, which includes a number of species derived from Mercury’s surface, offers a novel laboratory for exploring the nature of the complex and changing interactions among the solar wind, a small magnetosphere, and a solid planet. Recent ground-based astronomical measurements and several new theoretical developments set the stage for the in-depth exploration of Mercury by two spacecraft missions within the coming decade.