Introduction: The Female Sexual Function Index (FSFI) is a patient-reported outcome measure measuring female sexual dysfunction. The FSFI-19 was developed with 6 theoretical subscales in 2000. In 2010, a shortened version became available (FSFI-6). Aim: To investigate the measurement properties of the FSFI-19 and FSFI-6. Methods: A systematic search was performed of Embase, Medline, and Web of Science for studies that investigated measurement properties of the FSFI-19 or FSFI-6 up to April 2018. Data were extracted and analyzed according to COnsensus-based Standards for the selection of health Measurement INstruments (COSMIN) guidelines. Evidence was categorized into sufficient, insufficient, inconsistent, or indeterminate, and quality of evidence as very high, high, moderate, or low. Main Outcome Measures: The Main Outcome Measure is the evidence of a measurement property, and the quality of evidence based on the COSMIN guidelines. Results: 83 studies were included. Concerning the FSFI-19, the evidence for internal consistency was sufficient and of moderate quality. The evidence for reliability was sufficient but of low quality. The evidence for criterion validity was sufficient and of high quality. The evidence for structural validity was inconsistent of low quality. The evidence for construct validity was inconsistent of moderate quality. Concerning the FSFI-6, the evidence for criterion validity was sufficient of moderate quality. The evidence for internal consistency was rated as indeterminate. The evidence for reliability was inconsistent of low quality. The evidence for construct validity was inconsistent of very low quality. No information was available on structural validity of the FSFI-6, and measurement error, responsiveness, and cross-cultural validity of both FSFI-6 and FSFI-19. Clinical Implications: Conflicting and lack of evidence for some of the measurement properties of the FSFI-19 and FSFI-6 indicates the importance of further research on the validity of these patient-reported outcome measures. We advise researchers who use the FSFI-19 to perform confirmatory factor analyses and report the factor structure found in their sample. Regardless of these concerns, the FSFI-19 and FSFI-6 have strong criterion validity. Pragmatically, they are good screening tools for the current definition of female sexual dysfunction. Strength & Limitation: A strong point of the review is the use of predefined guidelines. A limitation is the use of a precise rather than a sensitive search filter. Conclusions: The FSFI requires more research on structural validity (FSFI-19 and FSFI-6), reliability (FSFI-6), construct validity (FSFI-19), measurement error (FSFI-19 and FSFI-6), and responsiveness (FSFI-19 and FSFI-6). Further corroboration of measurement invariance (both across cultures and across subpopulations) in the factor structure of the FSFI-19 is necessary, as well as tests for the unidimensionality of the FSFI-6. Neijenhuijs KI, Hooghiemstra N, Holtmaat K, et al. The Female Sexual Function Index (FSFI)—A Systematic Review of Measurement Properties. J Sex Med 2019;16:640–660.