 Cluster is a joint mission between ESA and NASA which was launched in 2000 to study the Earth's magnetosphere and the interaction between it and the Sun. It consists of four identical satellites which were launched in pairs in July and August 2000. These satellites have provided unprecedented insight into the structure and dynamics of the Earth's magnetosphere and its interactions with the Sun. This article was authored by C. P. Escobar, M. Ferringer, and M. Goldstein.