Task scheduling is a main activity in the design of real-time systems (RTS). It assures both functionality and safety of such systems. RTS can be modeled as a set of periodic tasks that must be completed before speci/c deadlines. In this paper, we investigate the fuzzy scheduling models on RTS and the main methodologies that solve these models. Thus, we present general periodic task scheduling models with fuzzy deadlines and fuzzy processing times; scheduling algorithms based on optimal assignment of the priorities; and a more general framework for designing RTS, Rate Monotonic Scheduling Theory, that includes the scheduling algorithms. A case study will illustrate the use of the theory. c © 2001 Elsevier Science B.V. All rights reserved.