This article describes a model-based development of a new nonlinear controller for control of Functional Electrical Stimulation, which can be used to restore movement in paralyzed individuals. The control design is based on the theory of sliding mode control. The controller is mathematically derived and shown to provide asymptotic stability of knee joint angle tracking by electrical stimulation of knee extensor muscle group only, or by electrical stimulation of knee extensor and flexor muscle groups. Its behaviour was evaluated in simulations with artificial and physiological knee joint angle reference trajectories. The controller was able to track trajectories with a period of 2 s with a root-mean-square error of approximately 2 degrees, which is considered a good performance. It was also shown to be robust to parameter variations of the model. This is important as models for different persons will differ considerably.