An analysis of over forty full scale impact tests has revealed much about the nature of side impacts and how improved protection can be achieved. Most of the tests have used the proposed EEVC Side Impact Test, which has been shown to be capable of distinguishing between cunent production cars which appear to offer different levels of protection. The most important factor identified as influencing protection is the vertical intrusion profile of the door. By separating the load path through the door from those into the car's structure, it has been possible to control door motion whilst at the same time increasing the rate of momentum transfer from the bullet vehicle to the target car. The increase in protection afforded by cars modified in this way has been substantial. The dynamic effects oftransient door motion and structural failure have been seen to be important. It is not yet clear whether adequate account can be taken of them in computer simulation modelling.