Compatibility is now generally recognized as the next big step forwards for car occupant secondary safety. The work performed to date has focused on the structural performance of vehicles, with the aim of providing a safe environment for the protection of the occupants in which intelligent restraint systems of the future could operate. 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 sti ness matching and passenger compartment strength. Recent advances in the development of a deformable barrier face for the full-width test to assess structural interaction, using high-resolution load cell wall measurements, are described. Analysis of the load cell wall data collected in EuroNCAP tests, to address the frontal sti ness problem, is reported together with initial work to investigate the repeatability of the passenger compartment strength test. In addition, for some of these tests, possible performance criteria are suggested. This research is being carried out in co-operation with the European Enhanced Vehicle-safety Committee and the International Harmonization of Research Activities Working Groups and is funded by the Department for Transport.