People aging with disabilities such as spinal cord injury (SCI) or multiple sclerosis (MS) report high rates of major depression. Depression frequently adds to the disabilities and suffering in these populations. Few definitive studies of depression treatments have been done in people with MS and none in SCI. There are several reasons to explore novel treatments for major depression in these groups. First, standard treatments, such as antidepressant medications, may not be as effective in people with neurological disabilities. Next, people with physical disabilities tend to be inactive. Lack of physical activity has been positively correlated with higher levels of depression. Longitudinal data and treatment trials suggest that increased physical activity is related to improved mood. Controlled trials show that increased exercise and physical activity can be effective treatments for major depression in nondisabled older adults. Previous research by the investigators' group suggests that people with MS are quite interested in exercise and that exercise is a safe and effective treatment for depression in younger, less disabled people with MS. Exercise may have widespread benefits for people with MS or SCI. Finally, exercise or increased physical activity represents a low cost, non-stigmatizing, highly accessible potential treatment for depression in people with physical disabilities. In this study the investigators will determine whether a relatively brief telephone-based intervention to promote physical activity is an effective treatment for major depression in people aging with MS or SCI. The investigators define "aging" as chronological age greater than 45 years old.