The key aim of this research was to provide an overview of the legislation, policy and practice concerning harm reduction services provided for problematic drug users (PDUs) in 9 European Union (EU) countries. To achieve this, the main objectives were to analyse international, national policies on harm reduction; to explore how harm reduction in conceptualised in different cultural contexts; to identify existing harm reduction initiatives in prisons; to identify the obstacles and barriers that need to be to overcome in order to implement harm reduction measures in prisons; to examine in detail the policies and harm reduction services in place in two sample institutions; to identify models of best practice and promote awareness of the harm reduction initiatives operating in the area of problematic drug users in custody, and finally to present examples of harm reduction measures in prisons from each of the sample countries. Key requirements needed in order to successfully implement sustainable harm reduction services in prisons are identified. These requirements can be formulated on all relevant levels: attitude towards and knowledge about harm reduction, drug addiction and health risks for all key actors, necessary changes both on the policy and practice level. Guidelines are formulated from evidence-based practice, as opposed to moral and value judgements.