This research contributes to the existing body of knowledge relating to crash compatibility (the minimisation of injury risk faced by all participants involved in a collision in traffic). The research focuses on the topic of structural interaction in collisions involving passenger vehicles, a phenomenon describing the efficiency of energy dissipation within existing deformation-zones of a passenger vehicle during a collision. A new definition for structural interaction was developed and several metrics to evaluate structural interaction and compatibility in car-to-car collisions were proposed, based on the commonly known Equivalent Energy Speed (EES) metric. The new EES metrics describe equivalent closing velocities for a given collision based on the energy dissipated within the front-ends (EESFF) and the entire structure (EESVV) of both vehicles involved in a head-on collision. These metrics form the basis of the new knowledge generated by this research. Additionally, a new method was developed to measure the amount of energy dissipated through structural deformation in a collision, based on accelerometer readings. This method was applied to several experimental and simulationbased car-to-car collisions and the validity of the method was proven. Based on the energy dissipation which occurred in the car-to-car collisions analysed, the degree of compatibility reached and the level of structural interaction which occurred in each collision was evaluated by applying the newly developed EESFF and EESVV metrics. Thie research also investigates the assessment of vehicles' structures in a standardized procedure with a view to improving structural interaction in the real-world. Several fixed barrier crash tests have been proposed in different configurations and with different assessment criteria. All assessments aim to evaluate the geometrical characteristics of the front-ends of passenger vehicles. A set of factors required from a compatilibility assessment focused on assessing vehicle geometry were identified. The proposed compatibility assessment procedures were evaluated based on their ability to predict the potential for structural interaction offered by passenger vehicles.