Attention-deficit/hyperactivity disorder (ADHD) is highly heritable. Confirmed association has been reported for several candidate genes, including DAT1, DRD4, SNAP-25, DRD5, 5HTT, HTR1B, and DBH; however, these confer relatively small risk. Family-based linkage studies have identified a number of chromosomal regions containing potential ADHD predisposing loci, some overlapping in two or more studies, including 5p, 6q, 7p, 11q, 12q, and 17p. New large-scale studies that apply recent technological advances to perform high-density genotyping of the entire genome, in combination with information on the haplotype structure of the human genome, now allow testing of single-nucleotide polymorphism association with disease phenotype without any a priori hypothesis. They may contribute further to our understanding of the genetic factors involved in ADHD. The heterogeneous complex ADHD phenotype, as well as epigenetic factors may be contributing to the challenge of genetic studies. Samples that include limited age ranges may have better success at uncovering genes whose expression is limited to specific developmental stages.