In this paper we study sliding mode controllers, in particular those intended for tracking. It is well known that a sliding motion occurs if si Â¿i ≪ 0 in the neighborhood of the sliding surface si = 0. Our studies appear to show that the guarantee of "stability" on the sliding manifold is quite delicate. Only simulation evidence was available at the time of writing this paper. We also give new results on sliding mode controllers by requiring Â¿i = -ki sgn si where si is the i-th switching surface and ki an appropriately chosen positive parameter. This choice makes the synthesis of sliding mode controllers quite simple. Several examples will be given to illustrate our findings on a potential instability and the efficacy of the latter approach.