Many jobs in industry severely tax the worker's musculoskeletal system and may approach or exceed the worker's maximum voluntary strength capabilities. When this occurs, there is evidence that the worker is at higher risk of experiencing a musculoskeletal disorder (Chaffin, 1978; Keyserling et al., 1980). It is for this reason that efforts have been taken over the last couple of decades to provide a means of evaluating the muscular strength capabilities of workers, so that jobs can be designed to eliminate taxing exertions, and to ensure that workers performing physically demanding jobs have the strength to safely perform required tasks. The effectiveness of worker strength evaluation in reducing work-related musculoskeletal disorders (WMSDs) depends in large part on the purpose of the evaluation. Assessment of physical strength has been used for two primary purposes in the field of ergonomics: job design and worker selection. Job design has been the focus of the psychophysical method of strength assessment and is the technique most likely to have a positive impact in reducing WMSDs. In this technique, the strength of a population of workers is used to design the job so that the majority of workers find the exertion to be acceptable. Studies have indicated that designing tasks by this approach may reduce back injuries by up to 33% (Snook et al., 1978). Strength testing has also been used for the purpose of worker selection, that is, making sure that workers have sufficient strength to perform physically demanding jobs. In a sense, this approach to controlling WMSDs in antithetical to one of the primary tenets of ergonomics: design the job to fit the worker. Instead, worker selection seeks to "fit" a strong worker into a physically demanding job. Predictably, this technique does not result in nearly the same magnitude of reduction in injuries compared to the job design approach. However, some studies have indicated a partial success using this technique. It should be noted that such an effect has only been evident when the procedure is employed in an environment known to place workers at very high risk of injury. Furthermore, it must be noted that most studies that have examined worker selection procedures have been short term (usually a follow-up period of one year or less). There can be no guarantee that this approach will be successful in protecting workers over the long term. Muscular strength is a very complex function that can vary greatly depending upon the methods of assessment (Gallagher et al., 1998). As a result, there is often a great deal of confusion and misunderstanding with regard to the appropriate uses of strength testing in the context of ergonomics. It is not uncommon to see techniques misapplied by those unfamiliar with the caveats and limitations associated with various strength-0testing procedures. The purposes of this chapter will be threefold: to provide the reader with a basic understanding of human strength, to characterize various methods of strength testing, and to describe ways that these techniques have been used in the attempt to control work-related musculoskeletal disorders (WMSDs).