The operation of the heavy ion storage ring ESR has been continuously improved. The energy range of decelerated highly charged ions could be extended down to 3 MeV/u. Electron cooling is available over the full energy range from injection to the final energy. Stochastic cooling has been established as a powerful pre-cooling method for fragment beams in combination with final electron cooling which allows a considerable reduction of the total cooling time. The isochronous mode of the focussing structure allows studies of the performance of electron cooling above transition energy. Experiments with a cooled beam above transition energy showed strong heating due to microwave instabilities.