The use of substitution drugs for the treatment of opioid dependence represents a key evidence-based response to heroin problems in Europe. However, the diversion of these medicines from their intended use in drug treatment to non-medical use and sale on illicit drug markets is a cause for concern. The use of diverted substances has been associated with fatal and non-fatal overdose and an increased incidence of opioid dependence. This analysis summarises the context of opioid substitution treatment in Europe, what diversion is and what current measures are being implemented to reduce it, and considers the implications for the future.