The chapter reviews the molecular genetics of Attention Deficit Hyperactivity Disorder (ADHD), highlighting the complex genetic architecture of the disorder. Despite twin studies indicating high heritability, genome-wide linkage and association scans have yielded inconclusive results. Candidate gene studies, however, have provided substantial evidence implicating several genes in ADHD etiology. Meta-analyses support the role of *DRD4*, *DRD5*, *SLC6A3*, *SNAP-25*, and *HTR1B* genes in ADHD. The literature also discusses the association of other catecholaminergic and serotonergic genes, such as *DBH*, *MAOA*, *SLC6A4*, *CHRNA4*, *GRIN2A*, *HTT*, *TPH*, *SNAP25*, *BDNF*, and *CHRNA4*. The chapter emphasizes the need for future studies to refine phenotypes, investigate comorbid conditions, and explore gene-environment interactions to better understand the genetic basis of ADHD.The chapter reviews the molecular genetics of Attention Deficit Hyperactivity Disorder (ADHD), highlighting the complex genetic architecture of the disorder. Despite twin studies indicating high heritability, genome-wide linkage and association scans have yielded inconclusive results. Candidate gene studies, however, have provided substantial evidence implicating several genes in ADHD etiology. Meta-analyses support the role of *DRD4*, *DRD5*, *SLC6A3*, *SNAP-25*, and *HTR1B* genes in ADHD. The literature also discusses the association of other catecholaminergic and serotonergic genes, such as *DBH*, *MAOA*, *SLC6A4*, *CHRNA4*, *GRIN2A*, *HTT*, *TPH*, *SNAP25*, *BDNF*, and *CHRNA4*. The chapter emphasizes the need for future studies to refine phenotypes, investigate comorbid conditions, and explore gene-environment interactions to better understand the genetic basis of ADHD.