The South-East Asia Region contains an estimated 400,000-500,000 people who inject drugs (PWID). HIV prevalence among PWID is commonly 20% or higher in Indonesia, Thailand, Myanmar and some regions of India. Opioid substitution therapy (OST) is an important HIV prevention intervention in this part of the world. However, key challenges and barriers to scale up of OST exist, including: pervasive stigma and discrimination towards PWID; criminalisation of drug use overshadowing a public health response; lack of political will and national commitment; low financial investment; focus towards traditional treatment models of detoxification and rehabilitation; inadequate dosing of OST; and poor monitoring and evaluation of programmes. Our review of local evidence highlights that OST can be successful within the Asian context. Such evidence should be utilised more widely to advocate for policy change and increased political commitment to ensure OST reaches substantially more drug users. Significance for public healthSeveral countries in the World Health Organization South-East Asia Region can be commended for introducing opioid substitution therapy (OST) to address the ongoing HIV epidemic among people who inject drugs (PWID). Local evidence shows OST is an effective drug treatment approach in the Asian context given sufficient technical and institutional support. However, despite much progress, the number of OST dispensing sites and recipients remains totally inadequate in terms of impact upon the current HIV epidemic among PWID. Ongoing advocacy is needed if countries are to achieve the WHO's target of 40% of PWID being dosed with OST. Greater political commitment a strengthened policy environment, capacity building for OST clinics, lessening the criminalisation of drug use and promoting a public health response will give many more PWID access to OST and slow the advance of the HIV epidemic.