Background: The latest generation of rear-end whiplash protection systems, as found in the WHIPS Volvo and SAHR Saab, have reduced injury rates by almost 50% in comparison with the previous generation of seat/head restraint systems. Occupant behaviour, such as seated posture and seat adjustment settings, may affect the injury risk. Method: Five studies were conducted. Studie I was an injury outcome study based on insurance data. Studies II-IV investigated seat adjustment, occupant backset, and cervical retraction for drivers and occupants in different postures and positions in the car, during stationary and driving conditions. Study V compared the occupant data from studies II and III with a vehicle testing tool, the BioRID dummy, using the protocols of the ISO, RCAR, and the RCAR-IIWPG. Results: Female drivers and passengers had a threefold increased risk for medically-impairing neck injury in rear-end impacts, compared to males. Driver position had a double risk compared with front passenger seat position. Female drivers adjusted the driver seat differently to male drivers; they sat higher and closer to the steering wheel and with more upright back support. The volunteers also adjusted their seat differently to the ISO, RCAR, and RCAR-IIWPG protocol settings; both sexes sat further away from the steering wheel, and seat back angle was more upright then in the protocols. In stationary cars, backset was highest in the rear seat position and lowest in the front passenger seat position. Males had a larger backset than females. Cervical retraction decreased and backset increased for both sexes when posture changed from self-selected posture to a slouched posture. The BioRID II dummy was found to represent 96th percentile female in stature, and a 69th percentile female in weight in the volunteer group. Conclusions: Risks in car rear-end impacts differ by sex and seated position. This thesis indicates the need for a 50th percentile female BioRID dummy and re-evaluation of the ISO, RCAR, and RCAR-IIWPG protocols, and further development of new safety systems to protect occupants in rear-end impacts.