Accuracy is the ubiquitous goal of dynamic simulation, in order to yield the "correct" motion. But for creating animation, what is really of interest is "plausible" motion, which is somewhat different. We discuss what we mean by plausible simulation, how it differs from "accurate" simulation, and whywe think it's a worthwhile area to study. The discussion touches on questions of physically plausible vs. visually plausible motion, plausible simulation in a noisy or textured environment, and probability measures for motion, as well as issues for forward and inverse problems. 1 Introduction Simulation is generally used in the context of a predictive model of behavior: given a precise description of a real-world situation, try to determine computationally what would really happen. When designing airplane parts, for example, accuracy of the model and of the simulation are critically important. However computer graphics animation has different needs, requiring a slightly different ou...