Motion information plays a key role in numerous image processing tasks. 2-D motion in imagery is induced by 3-D motion of projected objects or movements of the camera. Roughly spoken, from a signal-processing point of view, motion trajectories represent the spatio-temporal directions of the strongest intensity resemblance in image sequences. From a measurement point of view, 2-D motion in imagery is an observation incorporating information about 3-D geometry and 3-D motion of the real world.