Musculoskeletal disorders account for almost 70 million visits to physicians in the U.S. annually. About 1 million people take time off from work annually to treat and recover from work-related musculoskeletal pain or impairment in the lower back or upper extremities. The economic consequences can be measured by adding compensation costs, lost wages, and lost productivity; the total is usually between U.S. $45 and U.S. $54 billion annually (National Research Council, 2001). There has been considerable debate concerning the causes, nature, severity, and degrees of work-relatedness of musculoskeletal disorders as well as the effectiveness and cost-related benefits of various interventions. None of the common musculoskeletal disorders is uniquely caused by work exposures. They are what the World Health Organization calls “work-related” conditions because they can be caused by work exposures as well as non-work factors.