Abstract: This practical guide on “Opioid Substitution Treatment in Custodial Settings” is to support prison doctors, contracted doctors, prison health care workers, prison administration, NGOs and others in delivering or supporting substitution treatment to opioid dependent prisoners. Drug dependence has to be treated as a severe disease and everyone has a part to play to ensure the best treatment for prisoners and also to ensure that drug related harm is kept as low as possible. Applying the recommendations in this guide will contribute to a healthier prison for prisoners with drug dependence with satisfying roles for staff members and a marked reduction in the harm that drug use in prisons can create.