Fragile X syndrome type A (FXS or FRAXA) is the most common inherited cause of intellectual disability, with an estimated prevalence of 1/4000 to 1/6000. Nearly all cases of FXS are caused by the expansion of an unstable CGG repeat in the 5’ untranslated region of the FMR1 gene. Most individuals in the general population have 44 or fewer CGG repeats, while individuals with a full mutation causing FXS have more than 200 CGG repeats. Males with FXS exhibit mild to severe intellectual disability, often associated with autism-spectrum disorders, ADHD, speech and language delay, anxiety, and other characteristic behaviors. Physical features include macrocephaly and a distinctive facial appearance including large ears, a long face with a prominent forehead, prognathism, and a high arched or cleft palate. Macroorchidism may be observed in postpubertal males. Females with full mutations exhibit a broad spectrum of clinical presentations depending in part on the pattern of X inactivation. Approximately 50% of females with a full mutation have borderline or mild intellectual disability, and they also may exhibit shyness and anxiety. Some females with full mutations have more significant cognitive, behavioral, and physical features of FXS similar to those observed in males, while others have normal intelligence with only subtle learning disabilities. An estimated 1 in 259 females and 1 in 755 males in the general population have an FMR1 premutation with 55 to 200 CGG repeats. While they do not have FXS, premutation carriers have an increased risk for neurological, psychiatric, and physical disorders. Approximately 40% of male and 8-17% of female premutation carriers over the age of 50 report experiencing symptoms of fragile X-associated tremor/ataxia syndrome (FXTAS). FXTAS is a neurodegenerative disorder characterized by cerebellar ataxia, intention tremor, parkinsonism, progressive cognitive decline, and psychiatric disorders. Additionally, approximately 21% of female premutation carriers develop primary ovarian insufficiency (POI) leading to early menopause and infertility. Premutation carriers may also have an increased frequency of learning difficulties, depression, social anxiety, and other medical issues, although additional research is necessary to clarify the association between premutation alleles and these disorders.