Although it is assumed that genes that influence cognitive function are ubiquitous in the human genome, to date, more such genes have been found on the X chromosome than on any other comparable segment of the autosomes. This is in large measure because of the power of hemizygosity in exposing mutations of X-linked genes in males. Clinical manifestations, mapping of gene loci by linkage analysis or chromosome rearrangements, and gene identification by positional cloning or mutational analysis of candidate genes have permitted extensive lumping and splitting within the large and heterogeneous category of X-linked mental retardation (XLMR). Approximately 130 XLMR syndromes have been identified, 25 gene loci have been mapped and cloned, and 55 other loci have been mapped but not cloned. Well-recognized syndromes (e.g., Fragile X and Coffin-Lowry syndromes) and syndromes represented by only a single family (e.g., Arena and monoamine oxidase-A syndromes) are among these more or less well-defined entities. In addition, more than 75 families with nonsyndromal XLMR have been regionally mapped and 7 causative genes have been identified.