There is a broad range of substance-related problems that arise for the forensic psychiatrist. For many, what is most challenging about addressing these issues are the complex behavioral, biological, clinical, and social phenomena involved. Substance- related illnesses are on the cutting edge of brain research. Substance-related behavior that brings individuals to the attention of forensic psychiatrists involves a wide spectrum of substance use patterns, but even substance misuse may have profound and relevant effects, forensically. The social forces that mold our laws and attitudes toward addictive drug use are at work in almost every forensic context. Substance-related issues provide a rich medium for the application of forensic psychiatric principles and practice. As in all of forensic psychiatric work, the psychiatrist should be familiar with each forensic context in which addiction issues arise. They should become familiar with the relevant definitions, criteria, and legal requirements that apply in each specific area of their practice, rather than assume that clinical definitions and clinical reasoning will carry them. Comfort and effectiveness with addiction issues requires willingness to continually educate oneself about this rapidly changing field, and familiarity with one's own attitudes and beliefs regarding addictive illness.