The problems of mobility impairment in the elderly constitute new and major challenges for biomechanics research. This paper outlines what some of the important problems are, discusses the relevance of biomechanics research to these problems, and reviews some of the current state of knowledge about factors related to the biomechanics of mobility impairments in the elderly. The population of old adults is growing rapidly and the incidence of mobility impairments in old adults is high. Mobility impairment biomechanics research is needed to make the assessments of impairments more precise, to design therapeutic programs that are more effective and to learn more about how mobility impairments can be prevented.