The term “men who have sex with men” - frequently shortened to MSM - describes a behaviour rather than a specific group of people. It includes self-identified gay, bisexual, or heterosexual men, many of whom may not consider themselves gay or bisexual. HIV responses for transgender populations are also often considered alongside MSM initiatives.
MSM are often married, particularly where discriminatory laws or social stigma of male sexual relations exist. Largely because of the taboo, the female partners of men who have sex with men are often unaware of their partner's other liaisons, and may therefore be exposed to additional HIV risks. Forced sex among men is not uncommon, especially in men-only environments such as prison settings.

Sex between men occurs in every culture and society, though its extent and public acknowledgement vary from place to place. Sex between men is thought to account for between 5 and 10% of global HIV infections, although the proportion of cases attributed to this mode of transmission varies considerably between countries. It is the predominant mode of HIV transmission in much of the developed world.

There is the potential for rapid HIV transmission within populations of men who have sex with men, especially if the rate of unprotected anal intercourse is high. There is also high potential of prevention benefit of the programmes among men who have sex with men. However the coverage of the prevention has been low: where countries report on coverage, only around 40% of men who have sex with men have access to the HIV prevention and care services they need. Many factors contribute to this situation including denial by society and communities, stigma and discrimination, and human rights abuse. Also where prevention programmes are in place, potential increase in risk behaviours due to prevention fatigue should be taken into consideration on the programming.