This chapter discusses drug dependence, which includes addiction and habituation of drugs. Patterns of drug use disorders, DSM-IV criteria for diagnosis of substance dependence, psychoactive substances such as alcohol, amphetamines, nicotine, and opioids, complications of drug abuse, and postmortem findings, are some topics discussed in detail. The criterion for diagnosis of substance abuse disorders are hazardous or compulsive use, the role of impairment, and recurrent legal problems. Lysergic acid diethylamide (LSD) treatment includes removing the patient from aggravating situation, anxiolytics and symptomatic treatment. Complications of drug abuse may include vascular damage and gangrene, cellulitis and skin abscess formation, septicemia and subacute bacterial endocarditis, and psychiatric complaints. Postmortem findings include chronic edema of the hands, secondary to occlusive thrombophlebitis in the forearms, is seen occasionally in long-term addicts. Hyperplastic changes in the reticuloendothelial system are also common.