The European Spallation Source (ESS) is an intergovernmental project building amultidisciplinary research laboratory based upon the worlds most powerful neutron source. The main facility will be built in Lund, Sweden. Construction is expected to start around 2013 and the first neutrons will be produced in 2019. The ESS linac will deliver 5 MW of power to the target at 2.5 GeV, with a nominal current of 50 mA. The 120 high beta elliptical cavities, which operate at a frequency of 704 MHz and accelerate protons from 600 MeV to 2.5 GeV, account for more than half of the total number of RF cavities in the ESS linac and three quarters of the total beam power needed. Because of the large number of RF power sources and the high power level needed, all the design and development efforts for the RF power source have so far been focused on this part of the accelerator. The design and development status of the RF power source is reported in this paper with emphasis on reliability, maintainability, safety, power efficiency, investment cost and production capacity. (Less)