With increasing power generation out of renewable energy sources, there is a rising interest to investigate their impact on the power system and its control. In this paper, both the impact on frequency control and the capability to deliver frequency support by renewables is presented. A test grid is used to also investigate the variation of system inertia as a function of time. It is shown that by integrating renewables in the generation mix, the frequency support deteriorates, but through additional control, the frequency support can be improved. Finally the control of an inertialess grid is shortly described and some recommendations for future research are given