The use of psychoactive, potentially dependence-producing substances is highly prevalent around the world, and contributes substantially to global disease burden. There is a major gap between the need for treatment for substance use disorders. Changes proposed for the classification of substance use disorders in the Eleventh Revision of the International Classification of Diseases and Related Health Problems, based on a public health approach, have important implications for the conceptualisation, structure and availability of services. These include: (1) an updated and expanded range of substance classes; (2) greater specification of different harmful patterns of substance use, which may be continuous or episodic and recurrent; (3) a new category to denote single episodes of harmful use; (4) a category describing hazardous use of substances; and (5) simplification of diagnostic guidelines for substance dependence. This paper describes these changes and the opportunities they present for improved prevention, treatment, monitoring and health policy.