This chapter describes the major forms of drug use and dependence in developed countries, that is, cannabis, amphetamines, cocaine and heroin. It also presents the patterns of use, problems experienced by users and interventions to assist dependent users. Cannabis is the most widely used illicit drug globally, with around 150 million users, or 3.7% of the world's population aged 15 years and older. Regular cannabis smoking impairs the functioning of the large airways and causes chronic bronchitis. After cannabis, cocaine is one of the most widely used illicit drugs in developed and developing societies. According to the World Health Organization (WHO), amphetamines and methamphetamines are the most widely abused illicit drugs after cannabis, with an estimated 35 million users worldwide. Research in the United States indicates that dependent heroin users who seek treatment or come to attention through the legal system may continue to use heroin for decades.