UNAIDS considers gay men and other men who have sex with men, sex workers and their clients, transgender people and people who inject drugs as the four main key population groups. These populations often suffer from punitive laws or stigmatizing policies, and they are among the most likely to be exposed to HIV. Their engagement is critical to a successful HIV response everywhere—they are key to the epidemic and key to the response. Countries should define the specific populations that are key to their epidemic and response based on the epidemiological and social context. The term “key populations” is also used by some agencies to refer to populations other than the four listed above. For example, prisoners and other incarcerated people also are particularly vulnerable to HIV; they frequently lack adequate access to services, and some agencies may refer to them as a key population.