We observe that the switching activity at a circuit node, also called the transition density, can be extremely sensitive to the circuit internal delays.As a result, slight delay v ariations can lead to several orders of magnitude changes in the node activity.This has important implications for CAD in that, if the transition density is estimated by simulation, then minor inaccuracies in the timing models can lead to very large errors in the estimated activity.As a solution, we propose an efcient technique for estimating an upper bound on the transition density a t e v ery node.While it is not always very tight, the upper bound is robust, i n the sense that it is valid irrespective of delay v ariations and modeling errors.We will describe the technique and present experimental results based on a prototype implementation.