This paper presents a new approach to guaranteed state estimation for non-linear discrete-time systems with a bounded description of noise and parameters. The main result is an algorithm to compute a set that contains the states consistent with the measured output and the given noise and parameters. This set is represented by a zonotope. The size of the zonotope is minimized each sample time by an analytic expression or by solving a convex optimization problem. Interval arithmetic is used to calculate a guaranteed trajectory of the process state. Two examples have been provided to clarify the algorithm. (c) 2005 Elsevier Ltd. All rights reserved.