Sex chromosome aneuploidies comprise a relatively common group of chromosome disorders characterized by the loss or gain of one or more sex chromosomes. We discuss five of the better-known sex aneuploidies: Turner syndrome (XO), Klinefelter syndrome (XXY), trisomy X (XXX), XYY, and XXYY. Despite their prevalence in the general population, these disorders are underdiagnosed and the specific genetic mechanisms underlying their phenotypes are poorly understood. Although there is considerable variation between them in terms of associated functional impairment, each disorder has a characteristic physical, cognitive, and neurologic profile. The most common cause of sex chromosome aneuploidies is nondisjunction, which can occur during meiosis or during the early stages of postzygotic development. The loss or gain of genetic material can affect all daughter cells or it may be partial, leading to tissue mosaicism. In both typical and atypical sex chromosome karyotypes, there is random inactivation of all but one X chromosome. The mechanisms by which a phenotype results from sex chromosome aneuploidies are twofold: dosage imbalance arising from a small number of genes that escape inactivation, and their endocrinologic consequences.