Abstract: Recent estimates suggest that, each year, over 1 million Europeans receive treatment for problems related to the use of illicit drugs (EMCDDA, 2010). Of these, more than half receive opioid substitution treatment for problems related to opioid drugs, primarily heroin. The current levels of provision of drug treatment services in the European Union are the result of an expansion of services that has taken place during the last two decades. For much of that time, services expanded against a background of a growing drug problem, and one in which economic conditions were largely favourable. Now, however, in an era of cuts in government expenditure, managing the costs of treatment and ensuring the highest quality and best outcomes for the lowest possible cost are priorities. While measuring and improving drug treatment outcomes have been relatively high on the research agenda in recent years, the cost of treatment has received far less research attention. Addressing this information gap is likely to have a number of benefi ts for both decision-makers and treatment providers. On the one hand, service providers need accurate information on the costs of service provision in order to plan the allocation of resources. On the other, decision-makers and funders can use such information as a means of cost control, for example, by comparing costs of similar services or those of alternative providers of similar services. Finally, as a part of a fuller economic analysis, information on service costs is needed to determine the cost-effectiveness of interventions and treatment programmes. The cost of drug treatment in Europe is looked at from two main angles in this Selected issue. First, it presents an overview of the main sources of drug treatment funding and the financing mechanisms that are employed in European countries. The available data are presented on who is paying for drug treatment, how funding is organised, and how funding is distributed among different treatment modalities. This is followed by an analysis of the unit costs of different types of drug treatment. Costs related to opioid substitution treatment including the costs of substitution medication for a number of European countries are presented here. This publication keeps a tight focus on the cost of drug treatment and does not attempt to undertake a broader economic analysis of the healthcare and social costs relating to illicit drugs. This Selected issue represents a fi rst attempt to provide a European overview of drug treatment costs, and individual countries are brought into focus to illustrate specifi c aspects of the analysis. This subject is both topical and methodologically challenging. The varying availability of information as well as the complexity of funding arrangements mean that, at best, only very incomplete estimates can be made of the costs of (or expenditure on) drug treatment in Europe.