Substance abuse, including the use of alcohol and other drugs, is one of the most prevalent disorders among adolescents and young adults, co-occurring with many other psychiatric and mental health disorders. Some use of alcohol and drugs can be expected during adolescence; however, it is necessary to use screening tools to differentiate between misuse of drugs and alcohol and substance abuse. Some brief and comprehensive screening instruments are mentioned in this chapter for future reference. General indicators of substance abuse disorders such as physical and emotional symptoms, family issues, and social problems are explored, and the chapter discusses some of the most commonly abused drugs. Emphasis is put on incorporating treatments of substance abuse with treatments of mental illness for a more comprehensive intervention strategy for people with co-occurring disorders.