Attention deficit hyperactivity disorder (ADHD) is characterized by persistent and pervasive symptoms of inattention, hyperactivity, and impulsivity. ADHD is known as a ‘complex trait’ because of a phenotype with a genetic etiology that is composed of a multitude of susceptibility genes, each contributing only a small magnitude of the overall risk for the disorder. As so, this chapter aimed to establish a short review of the literature regarding to the genetics findings of ADHD. It is now well established that ADHD runs in families and is strongly genetically influenced. Also, studies have indicated a high heritability of ADHD (between 60% and 91%). Recent studies as the whole genome association (WGA) has showed an implicating specific susceptibility genes for ADHD. Regarding the genes in ADHD, studies have showed some candidate gene, related with the dopamine and serotonin genes. Also, it is important to highlight that the aetiology of ADHD, like all complex disorders, is not entirely explained by genes; environmental factors also contribute. So, in conclusion, all of the associated gene variants have been of small effect and in the future there will be increasing interest in examining the risk effects of multiple genes (gene-gene interaction) and interaction with environmental risk factors.