Rehabilitation biomechanics is a field of study that addresses the impact of disability and the effectiveness of rehabilitation therapies and interventions on human performance. Engineering and physics principles are applied to evaluate and analyze body movement and manipulation. This article describes biomechanical evaluation as a clinical and research tool for assessing the effect of disability on task performance, evaluating the risk of injury, and optimizing the fit of an assistive device to an individual. Methods and tools for measuring human kinematics, kinetics, energy expenditure, and muscle activity in rehabilitation applications are presented in addition to the use of biomechanical models and computer simulation as tools to determine parameters that cannot be measured with current technology or are difficult to measure directly. An introduction to gait analysis is reviewed with specific examples of cases where biomechanical analyses have led to new knowledge and improvements in clinical practices. Lastly, future trends and needs in rehabilitation biomechanics applications and research are discussed in relation to the increasing age of the population and persons aging with a disability.