This monograph (No. 06) reports on the systematic review of harm reduction. Harm reduction was defined as policies and interventions that focus on reducing the harms associated with drug use, not the amount of drug used. The following interventions were reviewed: needle syringe programs; supervised injecting facilities; non-injecting routes of administration; outreach; HIV education and information and HIV testing and counselling; brief interventions (aimed at harm reduction); overdose prevention interventions and legal and regulatory frameworks. There is substantial evidentiary support for NSP – as an efficacious, effective and cost-effective intervention. There is also good evidentiary support for outreach. The other harm reduction interventions (supervised injecting facilities, non-injecting routes of administration, overdose prevention, and brief interventions) do not currently have a sufficiently large body of research knowledge to draw strong conclusions. There is little evidence for HIV education and information and HIV testing and counselling as behaviour change interventions.