Fragile X syndrome is a genetic condition caused by a mutation, a cytosine-guanine-guanine triplet expansion, on the X chromosome. The behavioral phenotype includes characteristic developmental, cognitive, and psychiatric disorders, including childhood abnormalities of attention and social anxiety. Recent progress has been made in describing the molecular biology of the mutation, the neurocellular consequences of absent or decreased fragile X mental retardation protein, the behavioral phenotype, and the structural and functional brain abnormalities in fragile X syndrome. Recent research in fragile X syndrome studies has combined advances in molecular biology with a well-validated set of cognitive, adaptive, behavioral measures, and functional neuroimaging paradigms.