The possession, sale and use of certain substances is against the law, either completely or under certain circumstances, hence there are substance-specific offences, such as trafficking drugs, manufacturing certain chemical substances and selling alcohol to people under a specified age. Seen as rational if unwise choices, these activities are not within the purview of forensic mental health professionals. In mental health services, it is usually crimes that are in some way related to the individual’s intake of drugs that are the focus. In understanding the connection between substance use and crime, we first need to acknowledge the range of behaviours to which each of these terms refer. Substance use refers to a range of psychoactive substances, both legal and illegal, taken in a variety of ways, and with different effects on the individual. Crime also refers to a range of activities, with differing targets and varying levels of harm to others.