Control systems and methods for control of grid frequency in an electric power grid, are described. A grid frequency is monitored by monitoring devices at one or more predefined locations in the grid, and a determination is made whether one or more conditions relating to the monitored frequency have been met. A control period during which the grid frequency at one or more of the one or more predefined locations is to be controlled is initiated based on the determination. One or more variation characterizes relating to a variation, during the control period, in grid frequency are determined. Control instructions, comprising instructions to control power flow to and/or from each of a first plurality of power units so as to control the monitored frequency, are sent. The control instructions are generated on the basis of profile information relating to the power units and the determined one or more variation characteristics.