The European Drugs Strategy concentrates on demand reduction and supply reduction through international cooperation and research, information and evaluation. However, efforts to provide insight in the different aspects of Europe’s illicit drug problems have largely focused on indicators developed to assess demand-side strategies. The development of measures capturing dimensions of the supply of different illicit substances is an emerging field in the EU. To advance these efforts, the European Commission DG Justice, Freedom and Security commissioned a study to recommend indicators for improving the understanding of illicit drug markets, supply reduction efforts, and drug-related crime in the EU. In this study we reviewed literature and RAND’s previous work in this area, and we gathered insights from European experts and policymakers.