Purpose of Review: The relative influence of genes and environment on the liability to neurodevelopmental disorders (NDDs) can be investigated using a twin design. This review highlights the results of the most recent twin studies of NDDs. Purpose of Review: Recent twin studies have confirmed that NDDs show moderate-to-high heritability, and that from an etiological viewpoint both autism spectrum disorder (ASD) and attention deficit hyperactivity disorder (ADHD) are best regarded as the extremes on a continuous liability distribution. Both ASD and ADHD show high heritability in childhood and a substantial drop in heritability in adulthood, which is likely explained by the use of different assessment strategies in childhood versus adulthood, or by a complex mechanism of gene-by-environment interaction. NDDs show substantial comorbidity among each other, and with other mental health problems, which is partly because of a shared genetic etiology between different disorders. Summary: The findings of twin studies implicate substantial heritability of NDDs, and warrant large-scale molecular genetic studies for such traits. © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins.