Abstract. In this paper we solve tracking and disturbance rejection problems for stable infinitedimensional systems using a simple low-gain controller suggested by the internal model principle. For stable discrete-time systems, it is shown that the application of a low-gain controller (depending on only one gain parameter) leads to a stable closed-loop system which asymptotically tracks reference signals r of the form r(k) = N j=1 λ k j r j , where r j ∈ C p and λ j ∈ C with |λ j | = 1 for j = 1, . . . , N. The closed-loop system also rejects disturbance signals which are asymptotically of this form. The discrete-time result is used to derive results on approximate tracking and disturbance rejection for a large class of infinite-dimensional sampled-data feedback systems, with reference signals which are finite sums of sinusoids, and disturbance signals which are asymptotic to finite sums of sinusoids. The results are given for both input-output systems and state-space systems.Key words. discrete-time systems, disturbance rejection, infinite-dimensional systems, internal model principle, low-gain control, sampled-data control, tracking AMS subject classifications. 93C25, 93C55, 93C57, 93C80, 93D15, 93D25 DOI. 10.1137/080716517 1. Introduction. The synthesis of low-gain integral controllers for uncertain stable continuous-time plants has received considerable attention in the last thirty years. Let G be a stable proper rational continuous-time transfer function matrix. The main existence result for robust low-gain integral control states that if all of the eigenvalues of G(0) have positive real parts, then there exists ε * > 0 such that for all ε ∈ (0, ε * ), the controller (ε/s)I stabilizes G. Moreover, the resulting closed-loop system asymptotically tracks arbitrary constant reference signals. This result has been proved by Davison [2] using state-space methods and Morari [11] using frequencydomain methods. This low-gain controller allows stabilization and tracking with very little information about the plant, and it is not based on system identification. The above regulator result has been extended to various classes of (abstract) infinitedimensional continuous-time systems: in [12] for exponentially stable parabolic systems, in [7] for systems in the Callier-Desoer algebra (CD-algebra), and in [9] for exponentially stable regular systems.In the case that the reference and disturbance signals are of the form