Chapter 8 reviews empirical information concerning drug addiction and substance use disorders. Definitions are provided along with information concerning prevalence, multicultural, and developmental findings. The chapter discussed the initiation and maintenance of addiction based on reinforcement theories. Research concerning so-called reward pathways are critically reviewed, including mesocorticolimbic pathways. Information regarding the role of dopamine and other neurotransmitters are discussed. Morphological changes are also reviewed, including processes of cellular adaptation in response to substance use disorder. Etiologic theories are addressed, including heritability, twin, and family studies. Linkage and genome wide association studies are included as well as specific candidate genes. The role of various neurotransmitters in the addiction process is reviewed along with functional neuroimaging findings. Lastly, brain-based research findings concerning specific substances are discussed including alcohol, opioids, stimulants, and cannabis.