The NASA/European Space Agency (ESA) Cluster mission was the first to deploy four identical spacecraft flying in formation to conduct three-dimensional studies of the interaction between Earth's magnetosphere and the "solar wind" of charged particles streaming off the sun. Science output from Cluster greatly advances our knowledge of space plasma physics, space weather, and the sun-Earth connection, and has been key in improving the modeling of the magnetosphere and understanding its various physical processes. The quartet of spacecraft went into orbit in two launches, in July and August 2000.