This paper describes the design and development of the Hybrid III 10-year-old crash test dummy. The size of the dummy was chosen to fill the gap between the Hybrid III 6-year-old and the Hybrid III small adult female dummy which is also about the size of a 13-year-old teenager. Characteristic dimensions and segment weights of the dummy are based on the anthropometry of the average 10-year-old. Biofidelity response guidelines for forehead, sternum and knee impacts and for fore/aft neck bending are scaled from the midsize adult male biofidelity guidelines taking into account the effects of differences in size, mass and material properties due to the age difference. The dummy is similar in construction to the other Hybrid III dummies except it has an adjustable lumbar spine which allows the dummy to slouch and its neck structure is aligned with its thoracic spine. Data are given showing the responses of the prototype dummy relative to its biofidelity guidelines. The prototype dummy was evaluated in out-of-position air bag tests and three-point belt tests to evaluate its durability and ease of use. The prototype dummy has demonstrated acceptable biofidelity, repeatability and durability and the design has been approved by the SAE Hybrid III Dummy Family Task Group for production.