A controller/observer pair is presented, on the basis of sliding mode ideas, which provides robust output tracking of a reference signal using only measured output information. Closed-loop analysis indicates that asymptotic tracking of a constant reference signal will be achieved despite the presence of a class of matched uncertainty. Furthermore, a form of ’separation principle’ is shown to hold for this class of controller and observer, in the sense that they can be designed independently apart from a scalar function of the uncertainty bounds.