In this paper we review the aspects linked with the comorbidity of attention deficit hyperactivity disorder (ADHD). The disorders most frequently associated with ADHD are: Tourette syndrome, generalised development disorders, communication disorders, learning disorders, coordination development disorders, behavioural disorders, anxiety disorders, affective disorders and mental retardation. From the neurocognitive point of view, the executive functions play an important role in ADHD and from the neuroanatomical point of view there is involvement of the frontostriatal circuits. The functional model of ADHD based on these functions and these structures enables us to understand the comorbidity with the above mentioned processes. Given the high rate of comorbidity of ADHD it is important to identify the associated problems in order to rationalise the psychological approach and pharmacological treatment employed. We also review the therapeutic implications that comorbidity entails.