Electron Cyclotron Resonance (ECR) ion sources are used for many applications, ranging from the production of high intensity proton beams to the production of high charge state ion beams. This review is focused on the production of ions for heavy ion accelerators, which take advantage of the high reliability and stability of the beams produced by ECR ion sources as well as of their capability to produce currents in the order of some e&mgr;A of fully stripped light ions and of high charge states heavy ions. The improvement of performance through the years has been steady and an energy increase of the accelerated beams has followed. A brief summary of the main breakthroughs will be given, as the gas mixing and the electron donors. The role of the magnetic field and frequency will be described with a particular attention to the High B mode concept and to the method of multiple frequency injection. Some future developments that will increase the currents of the highest charge states and the design of some “3rd generation ECR ion sources” will be considered in the following. ©1999 American Institute of Physics.