Disruption mitigation is mandatory for ITER in order to reduce forces and to mitigate heat loads during the Thermal Quench (TQ) and from runaway electrons. A fast Disruption Mitigation Valve (DMV) has been installed at JET to study mitigation by Massive Gas Injection (MGI). Different gas species and amounts have been investigated with respect to timescales and mitigation efficiency. We discuss the mitigation of halo currents as well as sideways forces during vertical displacement events, the mitigation of heat loads by increased energy dissipation through radiation, the heat loads which could arise by asymmetric radiation and the suppression of runaway electrons.