The thesis solves the problem of finding an LTI controller that minimizes the steady-state tracking error of uncertain discrete-time systems. If a system is LTI, use of the "internal model principle" will assure that the error signal converges to zero. But this principle no longer applies when the physical plant is time-varying. This leads to the problem of how the steady-state value of the tracking error can be minimized in the time-varying case. The solution is provided in the following mathematical setting. The nominal model of the plant is SISO and LTI, and plant uncertainty is modeled by an arbitrary fading memory operator that is SISO, linear time-varying, and norm-bounded. A special case of the more general n-perturbation problem is also solved.