Purpose/Hypothesis: The purpose of this study was to examine the validity, reliability, and clinical value of manual muscle testing across physical therapy patient populations. Number of Subjects: 372 Materials and Methods: A systematic review of the literature was conducted, resulting in the identification of 9 articles that were relevant to the purpose of this study. These 9 articles were critically appraised via the Downs and Black scale as modified by Kennelly, which utilizes a series of questions to determine the level and quality of each article. Out of the nine articles this analysis tool qualified 3 of them to be fair (15-19); and 6 of them to be poor (>14). Results: Analysis of the nine identified studies provide multiple results of interest. One primary finding was that a manual muscle score of 3/5 and below was deemed reliable (both inter- and intra-rater reliability) when compared to hand-held dynamometry score. The overall validity and reliability of manual muscle testing across a variety of patient populations cannot be accurately determined due to the small number of articles that have been published. According to the multiple studies included in the systematic review, validity of manual muscle testing has shown to be poor. Alternatively, according to multiple studies included in the systematic review, interrater reliability of manual muscle testing has shown to be good. Lastly, only 3 of the 9 articles examined the clinical value of the data obtained from MMT and only for a limited number of the grades on the MMT scale. Conclusions: Overall, there is not enough evidence to determine if manual muscle testing is valid across all populations. However, it can be stated with confidence that scores of 3/5 and below are reliable due to of the objectivity of the score definitions, whereas grades 3+ and above should be considered to be subjective. Administers of the manual muscle test on patients with a graded score of greater than 3/5 on the 5-point scale or a 5/10 on Kendall’s 10-point scale may apply different amount of pressure to a patient when compared to other test administrators resulting in variation in grading and reduced objectivity. Clinical Relevance: Clinically, manual muscle testing is used widely in the practice of physical therapy. The result of this systematic review suggest usage of MMT should be limited when testing and retesting cannot be performed by the same individual due to poor interrater reliability. Manual muscle testing could still be used in most settings with the understanding that errors in interpretation and accuracy can occur. In addition, other techniques, such as hand held dynamometry (HHD), should be considered for increased validity and reliability across populations and between multiple raters. Finally, additional study is need to assess the clinical value of the data obtained from MMT in evaluation and treating patients.