Substorms, the frequent, brief magnetic disturbances that originate in Earth's magnetotail, cause acceleration of particles in the magnetosphere and their precipitation into the ionosphere, resulting in sudden brightening and increased movement and expansion of aurorae. Sergeev et al. review recent progress that has been made in understanding the onset and dynamics of substorms. In particular, recent observations have confirmed that magnetic reconnection—the breaking up and rejoining of magnetic field lines and release of energy—in the Earth's magnetotail, which sends fast plasma streams into the inner magnetosphere, is closely associated with substorm onset. Large-scale reconfiguration of the magnetic fields in the magnetotail and local instabilities interact in complex ways, leading to the onset of substorms.