The purpose of this study was to examine the concurrent validity of 4 clinical tests used to measure hamstring muscle length. A pilot study (N = 10) was conducted to determine the intratester reliability of 4 hamstring length measures: knee extension angle (KEA), sacral angle (SA), straight leg raise (SLR), and sit and reach (SR). The pilot investigation revealed good to excellent intratester reliability (intraclass correlation coefficient = 0.92-0.95) for each of the 4 tests. Eighty-one subjects (42 men and 39 women) participated in the main investigation. Subjects were randomly tested for each of 4 assessments of hamstring length. Concurrent validity was determined using linear regression, correlation, and kappa statistics. Correlation coefficients corresponding to the concurrent validity of the six combinations of the 4 clinical tests revealed poor to fair correlation (r = 0.45-0.65). The correlation coefficients for each pair from greatest to least were SR-SA= 0.65, SLR-SR = 0.65, KEA-SLR = 0.63, KEA-SR = 0.57, SLR-SA = 0.50, and KEA-SA = 0.45. Despite the common clinical use of these measures to assess hamstring length, these tests do not have sufficient concurrent validity to be used interchangeably or to assume that they each measure the same construct (hamstring length). Based on the results of this investigation and a review of the literature, the authors recommend that researchers, clinicians, and strength and conditioning specialists adopt the KEA test as the gold standard measure for hamstring muscle length.