ECRIS—Electron Cyclotron Resonance Ion Sources are able to feed accelerators with intense currents of highly charged ions. In ECRIS a high density—high temperature plasma is generated by means of the Electron Cyclotron Resonance Heating inside a B‐min, MHD stable trap. The state of the art about the principal heating mechanisms will be given. The paper will specially discuss the most critical and still open issues concerning the influence of the magnetic field and of the RF frequency on the plasma heating, as well as the impact of possible non‐linear pumping wave—to—plasma interactions. The contribution of INFN‐LNS will be specifically underlined. A short review on the future perspectives for the design of new generation ion sources will be given in conclusion.