Conventional approaches to dynamic scene analysis do not use motion itself explicitly for recognition. The authors propose a different approach for the use of motion in a computer vision system which uses the motion characteristics of moving objects without actually recovering the structure. In this approach, the extended trajectories followed by the objects are considered. It is argued that in many cases, where an object has a fixed and predefined motion, the trajectory of several points may serve to uniquely identify the object. In this approach, the trajectories are analyzed at multiple scales to identify important events corresponding to discontinuities in direction, speed, and acceleration using scale space. These important events are recorded in a presentation called trajectory primal sketch. Experimental results are presented graphically, demonstrating the potential value of this approach.>