This guide provides practical guidance on the use of Rapid Assessment and Response (RAR), a method specially designed to generate information that can help the planning and development of health policies and programmes as well as specific health interventions and services. The guide is designed for use by organizations and groups that wish to plan and carry out an assessment of the relationship between substance use and risk behaviour in the population of a particular area, in order to develop or expand the activities for promoting health and preventing harm, as well as to develop programmes for substance users, their families and social networks. The guide may also be used by those who are providing technical support to such organizations and groups. The guide places considerable emphasis on the risk of HIV infection associated with sexual behaviours in a wide range of situations of substance use - both non-injecting and injecting - including licit and illicit use, and dependent and recreational use.