This paper proposes and evaluates a new mechanism for I/O and network rate policing. The goal of the proposed system is to provide an simple, yet effective way to enforce resource limits on target classes of jobs in a system. The basic approach is useful for several types of systems including running background jobs on idle workstations, and providing resource limits on network intensive applications such as virtual web server hosting. Our approach is quite simple, we use a sliding window average of recent events to compute the average rate for a target resource. The assigned limit is enforced by forcing application processes to sleep when they issue requests that would bring their resource utilization out of the allowable profile. Our experimental results that show that we are able to provide the target resource limitations within a few percent, and do so with no measurable slowdown of the overall system. (Also cross-referenced as UMIACS-TR-2000-53)