A mathematical model of visual flow is presented, which could potentially account for an observer's ability to effectively interact with moving objects. The analysis demonstrates that there is visual information available to an observer about (a) a moving object's angle of approach, (b) changes in its velocity and acceleration, (c) whether its velocity and acceleration are positive or negative, (d) its time to collision with both the horizontal and vertical axes, and (e) whether it will cross the horizontal axis in front of or behind the point of observation. Several experiments are reported in which observer's sensitivity to these different aspects of visual information is examined using a variety of experimental tasks. The results suggest that human observers are highly sensitive to many abstract properties of visual stimulation, but that they are not sensitive to all of the information that is potentially available.