Missions to Mercury are challenging because of the planet’s proximity to the Sun, as close as one-third the mean Earth–Sun distance. This location imparts a stressing thermal environment because of intense solar illumination, as well as major propulsion requirements because of the energy gained by a spacecraft descending from Earth into the Sun’s gravity well. Although Mercury has been a primary exploration target since the 1960s, it was not until the discovery of gravity-assist trajectories to Mercury that robotic exploration became feasible. The Mariner 10 flybys in the 1970s revealed many of Mercury's characteristics and whetted the appetite of the science community for an orbiter mission. Enabled by multiple planetary gravity assists and innovations in spacecraft and instrumentation, MESSENGER successfully orbited Mercury from 2011 into 2015 and revolutionized our understanding of the planet. New questions raised by the MESSENGER results motivate the much larger, dual-spacecraft BepiColombo mission, scheduled to arrive at Mercury in late 2025. Even after BepiColombo, many key questions central to understanding Mercury’s formation will likely require a Mercury lander mission, potentially enabled by sufficiently large launch vehicles. The return of samples from Mercury to Earth may long remain an aspiration for future generations of scientists and engineers.