The purpose of this article is to provide an overview of the behavioral phenotype of FMR1 mutations, including fragile X syndrome (FXS) in order to better understand the clinical involvement of individuals affected by mutations in this gene. FXS is associated with a wide range of intellectual and behavioral problems, some relatively mild and others quite severe. FXS is the most common cause of inherited intellectual disability and one of the most prevalent genetic causes of autism spectrum disorder. Learning difficulties, attentional problems, anxiety, aggressive behavior, stereotypies, and mood disorders are also frequent in FXS. Recent studies of children and adults have identified associations between FMR1 premutation and many of the same disorders. We examine the neurobehavioral phenotypes of FXS and FMR1 premutation as they manifest across the lifespan of the individual. © 2010 Wiley-Liss, Inc.