Distinct groups of people within a society who share certain characteristics such as age, gender, geographic location, socioeconomic status, cultural background, or exposure to particular risk factors that may influence patterns of substance use, vulnerability to substance use disorders, and access to services. In the context of substance use systems, identifying and understanding different populations helps guide the design of prevention, treatment, recovery, and policy responses that are appropriate, equitable, and responsive to specific needs. Examples may include youth, women, people experiencing homelessness, individuals in the criminal justice system, or communities affected by specific social or economic conditions.