Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by deficits in social communication and social interaction, and restricted, repetitive patterns of behavior, interests, or activities. ASD is considered to be a multifactorial disorder, the risk of which can be elevated by various hereditary causes ranging from copy number variants to monogenic disorders. Fragile X syndrome (FXS) is a neurodevelopmental disorder that is thought to be the most common inherited cause of intellectual disability and the most common monogenic cause of ASD. This chapter highlights the phenotypic, neuroanatomical, and pathophysiological similarities and differences between idiopathic ASD and ASD as the result of a known genetic syndrome, and ways in which clinicians can diagnose and manage these disorders.