Opioids make the single largest contribution to illicit drug-related mortality and morbidity worldwide In this paper we reflect upon what has been learnt regarding treatment outcome and the natural history of heroin use from the Australian Treatment Outcome Study (ATOS). We focus on what we knew prior to ATOS, what ATOS revealed that is novel, and the implications for research, practice and policy. ATOS provided strong evidence for sustained improvement attributable to treatment across the three years of the study. It is argued that treatment for heroin dependence is money well spent, and leads to clear and sustained benefits to both heroin users and society.