Automobile accident reconstruction and vehicle collision analysis techniques generally separate vehicle collisions into three different phases: pre-impact, impact and post-impact. This paper will concern itself exclusively with the modeling of the impact phase, typically defined as the time the vehicles are in contact.Historically, two different modeling techniques have been applied to the impact of vehicles. Both of these techniques employ the impulse-momentum formulation of Newton's Second Law. The first relies exclusively on this principle coupled with friction and restitution to completely model the impact. The second method combines impulse-momentum with a relationship between crush geometry and energy loss to model the impact. Both methods ultimately produce the change in velocity. ΔV, and other pertinent information about a collision.The concepts of impulse-momentum and energy loss as applied to vehicle collisions have been occasionally misrepresented and appear not always to be fully understood. This paper will present the application of these principles to collisions of two bodies in a plane. This relationship between the change in velocity and energy loss will be investigated. A review and numerical comparison of several impact models will be presented.