Any reference on the genetic etiology of ADHD is widely accepted. Based on the evidence of today, this is as obvious to ADHD, such as for schizophrenia or bipolar disorder. Molecular studies have made important progress in terms of ADHD. For any genetic result, must be done multiple replication before it is accepted. 1) ADHD is hereditary. About 70% of the determination of this disorder is attributed to genes; 2) Studies on adopted children indicates that ADHD is on the line of the biological families rather than of foster families; 3) There is a strong association between ADHD and dopamine D4 receptor gene (DRD4). Similar results, but not so significant, were found for dopamine transporter gene (DAT1); Molecular genetic research, even though incipient, bring an important contribution to understanding the etiology of ADHD, and the results obtained so far, require replication in multiple studies to provide clear arguments.