Autism spectrum disorder (ASD) and attention deficit hyperactivity disorder (ADHD) regularly co-occur with one another. This article considers why this is the case. Recent research, particularly twin studies, has questioned whether overlapping genetic and environmental influences cause ASD and ADHD. These studies have consistently reported that a considerable degree of the genetic influences on ASD and characteristic traits overlap with those on ADHD. The implications of these findings for molecular genetic research and for clinical practice are considered.