Addiction is a highly prevalent brain disease. It is a major cause of many secondary forms of medical illness and accidents, and it is a leading root cause of death. The disease attacks the circuits of the brain that govern motivational learning and control. It is defined by increasingly compulsive drug seeking and use, despite the accumulation of negative medical, social, and psychiatric consequences. Because the disease also impacts brain systems governing the exercise of free-will, decision-making, and insight, it is often judged, criminalized, and stigmatized, which are countertherapeutic social responses to the disease. Addiction psychiatry is a field of psychiatry that is uniquely trained to treat the entire spectrum of addictions and mental illness, especially for mainstream dual-diagnosis patients who suffer with combinations of these disorders. The epidemiology of addiction shows that the disease is not evenly distributed in the population. Rather, it tends to concentrate in people with genetic, developmental, and environmental risk factors, many of which overlap with those that also produce mental illness. Advances and growth in addiction psychiatry training, research, and clinical care hold tremendous potential for ending mass incarceration and rendering the healthcare system more efficacious and cost-effective.