Compatibility is now recognised as the next major step forward for improved car occupant secondary safety. The work reported here forms part of a research project that was undertaken to understand the current compatibility problems in car to car collisions and develop crash evaluation procedures that are suitable for consumer and legislative testing. The research performed to date has focused on the structural performance of vehicles in order to provide a safe environment for the protection of the occupants. This should also provide sufficient space to allow intelligent restraint systems of the future to operate effectively. This paper outlines the present understanding of compatibility for frontal impact collisions and reports the current state of development of three possible test procedures to address the fundamental issues, namely:​ structural interaction, frontal stiffness matching and passenger compartment strength. The development of a new deformable barrier face and revised performance criteria for the full width test to assess structural interaction are described. Analysis of the load cell wall data collected in EuroNCAP tests, to address the frontal stiffness problem, is reported. Performance criteria are suggested and future work necessary to help set performance limits outlined. Initial work to investigate the repeatability of the passenger compartment strength test and possible performance criteria are described. This research is being carried out in co-operation with the European Enhanced Vehicle-safety Committee (EEVC) and the International Harmonisation of Research Activities (IHRA) Working Groups, and is funded by the UK Department for Transport (DfT).