The observation of the universe, the Earth monitoring, the docking of a capsule on the ISS, all these things require a certain attitude to carry out their tasks, so a robust attitude controller should be designed. The following thesis describes the design of a Sliding Mode Control (SMC) for a CMG (Control Moment Gyro) based testbed. A testbed is a platform for performing rigorous and replicable testing of scientific theories, new technologies, calculation tools and many control law is being developed on it. The testbed is driven by four CMGs which are torque generators used to perform attitude manoeuvre thanks to angular momentum conservation.