Gay, bisexual, and other men who have sex with men (MSM) have been among the most affected populations by HIV since the AIDS pandemic was first identified in the 1980s. Evidence from a wide range of studies show that these men remain at the highest risk for HIV acquisition in both developed and developing countries, and that despite three decades of evidence of their vulnerability to HIV, they remain under-served and under-studied. Prevention strategies targeted to MSM are markedly under-funded in most countries, leading to limited access to health services including prevention, treatment, and care. We explore the global epidemic among MSM in 2008, the limited funding available globally to respond to these epidemics, and the human rights contexts and factors which drive HIV spread and limit HIV responses for these men.What do we mean by the term MSM? MSM is a construct from the 1990s that tries to capture behavior and not identity. It was crafted to avoid stigmatizing and culturally laden terms such as gay or bisexual, which do not capture the wide diversity of orientations, sexual practices, cultures, and contextual settings in which male same-sex behaviors occur, and where HIV transmission and acquisition risks are centered. MSM includes both gay and non-gay identified men, bisexual men, and MSM who identify themselves as heterosexuals. It also includes men engaging in "situational" sex between men, such as can occur in prisons, schools, militaries or other environments; and it includes male sex workers who may be of any orientation but are often at very high risk for HIV. MSM may include some biologically male transgender persons, though some do not identify as male. And MSM includes a wide array of traditional and local terms worldwide-with enormous cultural diversity in Asia, Africa, Latin America and elsewhere. We use the term MSM here at its most inclusive.