This chapter provides a review of studies on the prevalence of Fragile X Syndrome (FXS) and factors that may lead to its variation, such as sex of the carrier, race/ethnicity, and genotype. We focus on the most recent systematic review and metaanalysis of the literature that was performed by Hunter et al. [Hunter, J., Rivero-Arias, O., Angelov, A., Kim, E., Fotheringham, I., & Leal, J. (2014). Epidemiology of fragile X syndrome: a systematic review and meta-analysis. American Journal of Medical Genetics A, 164A(7), 1648–1658] to provide estimates of the prevalence of FXS in males and females. We also document studies among various racial/ethnic groups and geographic regions to show that FXS is present in almost all subpopulations investigated. Lastly, we review the more rare sequence mutations in the FMR1 gene, specifically deletions and point mutations that lead to FXS-associated symptoms. We conclude by suggesting that additional large-scale studies are needed to better define the burden of the FMR1 mutations globally.