Countries experiencing concentrated epidemics of HIV need the size of key populations (KPs) to guide the national response on HIV and AIDS. Conducting a robust method to estimate the size of KPs is quite challenging as most of them are hidden and do not want to disclose theiridentity due to stigma and discrimination associated with their behaviour. KPs in Bangladesh include female sex workers (FSW), people who inject drugs (PWID), men who have sex with men (MSM) including transgender (TG)/Hijra and sex workers, and clients of sex workers in the country or abroad.