The understanding of vehicle and occupant motions is at the heart of vehicle accident reconstruction. Testing is frequently done to understand, verify and demonstrate accident motions and dynamics. Tests conducted with scale models can give very useful results at a fraction of the cost of full-scale testing. The test results can be used to predict and understand the motions and dynamics of the full-scale vehicles and occupants. The proper design, construction and testing with scale models and the interpretation of the test results is governed by the principles of similitude. This paper presents an introduction to the principles of similitude. These principles have been the basis for scale model testing of structures and machines by engineers for many years. This paper presents the principles of similitude and shows how these can be applied to a vehicle which becomes airborne. The results of these tests are compared with mathematical predictions of vehicle motions. The paper discusses the various types of motions which can be modeled in this fashion.