Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterized by concentration problems, hyperactivity and impulsivity. Disturbances in dopamine and/or noradrenalin neurotransmission are probably the underlying pathophysiological mechanisms of ADHD. Around 80% of variants of the phenotype can be ascribed to hereditary factors. There are various chromosomal loci containing ADHD genes. They partially overlap the loci found in linkage studies on dyslexia and autism. It seems likely that a number of genetic variants, each with a small effect size, in combination with gene-environment interactions predispose to ADHD. There is a high degree of phenotypical heterogeneity among people with ADHD. Finding endophenotypes may improve the power of genetic studies. Endophenotypes are specific expressions of the underlying pathophysiology, intermediate between gene and phenotype. Neuro-imaging studies in children with ADHD have indicated abnormalities in frontostriatal, temporal and cerebellar volume. Unaffected brothers and sisters show the same cerebral abnormalities, but not the cerebellar abnormalities. These brain abnormalities together with specific neuropsychological features could be ADHD endophenotypes.