Mercury is the innermost planet of the solar system and the eighth in size and mass. Its closeness to the Sun and its smallness make it the most elusive of the planets visible to the unaided eye. Until the last part of the 20th century, Mercury was one of the least-understood planets, and even now the shortage of information about it leaves many basic questions unsettled. Indeed, the length of its day was not determined until 1960s, and, even after the flybys of the Mariner 10 probe, the surface of the planet had not been completely observed. In 1990s, the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) developed two independent missions to Mercury. MESSENGER (MErcury Surface, Space ENvironmentm GEochemestry, and Ranging), selected by NASA in July 1999 has been orbiting about Mercury since March 11, 2011. The ESA dual-spacecraft mission BepiColombo will be inserted in orbit about Mercury in 2022. Both missions aim to determine Mercury’s deep interior that is a crucial information to understand the formation and evolution of the solar system.