The consistent and proper use of restraint systems by infants and children in passenger vehicles prevents hundreds of deaths and thousands of injuries each year. Misuse or improper selection of child restraints as well as nonuse by a small minority of children leads to many of the fatalities that do occur. Infants require the highest level of special treatment, with restraint systems designed to apply crash forces along the full length of their bodies. Toddlers can also benefit from rear-facing restraints. All children are best protected by harnessed restraints that snugly conform to their small body shape and are tightly installed in the vehicle. Belt-positioning boosters improve posture and belt fit so the vehicle seat belts can effectively protect older children in crashes. Seatbelts can provide good protection for children approaching the size of adults if the lap belt fits so it loads the pelvis and the shoulder belt fits so it loads the clavicle. Understanding both the theory behind the design of restraint systems and its application to child restraints is needed to develop improved restraint systems, and to provide informed guidance concerning child restraint selection and use.