Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder affecting 5.29% of children worldwide. It presents a heterogeneous clinical expression, and both environmental and genetic factors are involved in the etiology. Despite high heritability estimates, identification of genes that confer susceptibility to ADHD has been a slow and difficult process. The first genetic studies targeted dopaminergic genes, but the effects were small and only explained a small portion of ADHD heritability. Recent studies focus on the identification of novel genes and pathways that may underlie ADHD. The main goal of this review is to present evidence from genome-wide association, copy number variation and family-based studies of genetic susceptibility to ADHD. The challenges involved to disclose ADHD susceptibility genes will be reviewed in order to provide directions for future studies.