This project deals with the design and tuning of a wind farm controller, with focus on the active power and frequency control. A model of a 160 MW wind farm, containing 80 wind turbines is implemented to verify the controller functionalities. The requirements of the controller are based on the Danish grid codes. A PI-controller has been designed using the Modulus Optimum criteria and implemented into a main controller. The robustness of the controller and disturbance rejection capability has been verified successfully for dierent operating conditions. Two dispatch methods are investigated: equal and proportional to the available power. The latter is chosen due to its superior performance. The impact of the controller sampling time and discretization method on the performance is investigated both in an on-line simulation and in a Real-Time environment using OPAL-RT. The outcome of this project is a stepwise method to design and tune the active power controller.