MESSENGER’s exploration of Mercury has revealed a rich and dynamic geological history and provided constraints on the processes that control the planet’s internal evolution. That history includes resurfacing by impacts and volcanism prior to the end of the late heavy bombardment and a subsequent rapid waning of effusive volcanism. MESSENGER also revealed a global distribution of thrust faults that collectively accommodated a decrease in Mercury’s radius far greater than thought before the mission. Measurements of elemental abundances on Mercury’s surface indicate the planet is strongly chemically reduced, helping to characterize the composition and manner of crystallization of the metallic core. The discovery of a northward offset of the weak, axially aligned internal magnetic field, and of crustal magnetization in the planet’s ancient crust, places new limits on the history of the core dynamo and the entire interior. Models of Mercury’s thermochemical evolution subject to these observational constraints indicate that mantle convection may persist to the present but has been incapable of significantly homogenizing the mantle. These models also indicate that Mercury’s dynamo generation is influenced by both a static layer at the top of the core and convective motions within the core driven by compositional buoyancy.