MESSENGER was the first spacecraft to visit the planet Mercury in more than three decades and the first to orbit the solar system’s innermost planet, and it provided the first global observations of Mercury’s surface, interior, exosphere, magnetosphere, and heliospheric environment. This chapter begins with summaries of the objectives for the MESSENGER mission and the design of the spacecraft, payload instruments, and orbit selected to achieve those objectives. We then describe the procedures adopted to optimize the scientific return from the complex series of orbital data acquisition operations that MESSENGER followed. An overview is given next of the primary MESSENGER mission, including the three Mercury flybys prior to orbit insertion and the first year of Mercury orbital observations. We then outline the rationale for and accomplishments of MESSENGER’s first extended mission, conducted over the second year of orbital operations, and MESSENGER’s second extended mission, conducted over the final two years of orbital operations. The second extended mission included a distinctive low-altitude campaign completed at the culmination of the mission. A concluding section briefly introduces the other chapters of this book.