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.