Physical activity is extremely important in maintaining good health. Activity is not possible without a certain amount of flexibility. This report discusses issues related to flexibility fitness. Flexibility is a property of the musculoskeletal system that determines the range of motion achievable without injury to the joints. Static flexibility tests measure the limits of the achievable motion, but these limits are subjective. Dynamic flexibility tests are more objective and measure the stiffness of a passively stretched muscle group. However, there are no recommended field tests available at this time. Normal ranges of static flexibility are well-documented for most joints. Major deviations from the norm may be associated with a higher incidence of muscular injury. While there is theoretical association between flexibility and several musculoskeletal problems, there are few prospective studies showing significant associations. Currently, there is little scientific evidence upon which to base individual prescriptions for static flexibility development beyond the maintenance of normal levels. Any recommendation for stretching to improve flexibility should be based on a valid assessment of flexibility using sound testing procedures. Recommendations for stretching procedures based on recent reviews of the viscoelastic response of muscle to stretching are presented. (Contains 85 references.) (SM)