The physics studies of the three "heating" systems that are installed on JET are reviewed. Results from the beginning of JET up to now are presented with some emphasis on the more recent ones. The systems were used not only for heating, where JET has laid the groundwork to qualify them for heating the next generation of machines to ignition, but also increasingly as tools to control the plasma. This role, already exploited on JET, will become more and more important in the next machine, since main heating will be provided by the alpha-particle heating and one will have to rely on the heating systems to control the plasma during the burn.