The Pompidou Group has advocated the systematic and routine collection of information on patients entering treatment for problem drug use (treatment demand data) since the mid-1980s. Two decades later, the question now being asked is whether or not this data has been used as evidence in the development of policies and practices. In this publication, authors Hamish Sinclair, Carlo Bertorello, Michela Rial and Dusan Nolimal, all members of the Pompidou Group research platform, attempt to answer this question. Three case studies describe how treatment demand data has been used in the development of drug policies and services in Ireland, Italy and Slovenia. One strong message coming out of this report is the need for more information on the outcome of treatment. Policy makers clearly need more information on patients at the end of their treatment, including information on further treatment and its effectiveness.