The work reported here forms part of a research project that is being undertaken to further the understanding of compatibility in car to car collisions and develop crash evaluation procedures that are suitable for consumer and legislative testing. For frontal impact, full scale crash testing, accident analysis case studies and supportive finite element modelling studies have been used to identify the major factors that influence compatibility. One result is that the geometrical interaction of car structures has a large effect and it is now believed that obtaining good structural interaction is an essential prerequisite for frontal impact compatibility. Having achieved this, the next step is to control the global stiffness of the cars to ensure that they are able to absorb the collision energy, with minimal occupant compartment intrusion, without compromising the vehicle’s deceleration pulse profile. Frontal impact evaluation procedures are being developed, which use load cell wall measurements to assess a car’s compatibility. The current state of development of possible procedures is described, with an emphasis on results from full width deformable barrier tests. Procedures to assess side impact compatibility may be added following further research. This reported research is being used to support the European Enhanced Vehicle-safety Committee (EEVC) and the International Harmonisation of Research Activities (IHRA) Compatibility Working Group activities, and is funded by the Department of the Environment, Transport and the Regions (DETR).