Abstract : Work on improving gradient-based methods for optical flow estimation has been completed. An understanding of how errors arise makes it possible to define the inherent limitations of the gradient-based technique, obtain estimates of the accuracy of computed values, enhance the performance of the technique, and demonstrate the informative value of some types of errors. Significant results have been achieved on the problems associated with motion-based segmentation. An approach based on understanding the three-dimensional scene structure leading to an edge in optical flow has been developed. As a result, it is possible to simultaneously detect edges and determine important three-dimensional properties of the associated scene surfaces. The methods which have been developed make it possible to distinguish between occluding and occluded surfaces at a boundary. This technique may make it possible to link image regions corresponding to a partially occuled object and to produce descriptions of object boundaries that are less affected by occlusion. In addition, being able to distinguish between occluding and occluded boundaries is a crucial step towards determining the three-dimension position of surfaces. Keywords: image understanding, visual motion.