In support of the European Enhanced Vehicle-safety Committee (EEVC) research program and through it, the International Harmonization of Research Activities work on compatibility, TRL is investigating the compatibility of cars in frontal and side impact scenarios. Initial research has focused on identifying the major factors which influence compatibility and determining the extent to which they might influence injury outcome. Experimental crash test research is backed with Finite Element simulation modeling. For frontal impacts, full-scale testing has been used to examine the influence of vehicle mass, stiffness, structural interaction and geometry. The modeling work has studied how non-contact, deceleration-related injuries might be minimized by optimizing the deceleration pulse. For side impact, full car finite element models have been used for parametric studies to aid our understanding of the effects of the bullet vehicle mass, geometry and stiffness and to help predict more compatible designs. This has been backed by full-scale crash testing, aimed at determining the ideal characteristics of interacting car front and side structures. All of this work is aimed at developing crash test requirements that are capable of assessing a car''s compatibility.