This guide provides step-by-step guidance on the development of a national substance use treatment information system. The goal is to generate data on the individual characteristics and patterns of substance use among people in contact with the substance use treatment system, enabling treatment estimates of problematic substance use and the monitoring of treatment programmes. It introduces a minimum data set as the basis for data collection based on a core set of indicators. The guide is for policy-makers, public health experts, mental health and substance use specialists, and all others involved in substance use management, programming and planning. It will be useful in countries that wish to establish a substance use treatment information system or want to improve their current system.