Substance use disorders (SUDs) affect millions of people worldwide, with marked health and societal consequences. This chapter will provide a comprehensive overview of the epidemiology of substance use and SUDs, including regional and demographic variations in patterns of use. The multifaceted impact SUDs have on health outcomes and socioeconomic functioning will also be examined. Quantifying the burden imposed by SUDs, in addition to highlighting trends by geography and substance type, may boost continued public health efforts to provide targeted treatment.