This paper presents an overview of NHTSA’s vehicle compatibility research activities. NHTSA is monitoring the changing vehicle mix in the U.S. fleet, analyzing crash statistics, and evaluating the possible effects that these changes may have on vehicle crashes in the U.S. and thus on occupant safety. NHTSA is conducting full scale crash testing to develop a better understanding of vehicle compatibility and to identify test methods to assess vehicle compatibility. All of this research is being conducted with the close cooperation of the International Harmonized Research Activities compatibility research group.