Attention-Deficit/Hyperactivity Disorder (ADHD) is a neurodevelopmental disorder with symptoms from preschool age. More than half of children with ADHD show persistent symptoms and impaired function into adulthood. A neuropsychological paradigm and neuroimaging methods were used to investigate adults with ADHD. The results indicate impaired cognitive control, but normal auditory lateralization and normal ability to direct attention. An altered microstructure in the posterior part of the Corpus Callosum (CC) may reflect a delayed and only partial maturation of the CC, or a compensatory mechanism. Reduced Glutamate/Creatine ratio in the midfrontal region measured with magnetic resonance spectroscopy may reflect a disturbance of glutamatergic activity. In ADHD and other neurodevelopmental disorders the normal brain development is disturbed due to factors of genetic, epigenetic and environmental origins. A glutamatergic dysfunction with imbalance in the glutamatergic-catecholaminergic networks, may result in a spectrum of various phenotypes and endophenotypes, observed as different symptoms, as well as structural, functional, and neuropsychological abnormalities or impairments.