This 2011 primer published by the MSMGF is intended to serve as a guide on information related to men who have sex with men (MSM) for physicians, nurses, and other health care providers delivering HIV and primary care services in diverse clinical settings. Longstanding evidence indicates that MSM experience significant barriers to quality health care due to widespread stigma against homosexuality in mainstream society and within health systems. Social discrimination against MSM, or homophobia,a has also been described as a key driver of poor health outcomes in this population across diverse settings. In nearly every country around the world, MSM are disproportionately burdened with HIV and other sexually transmitted infections (STIs) when compared to the general adult population. Higher rates of depression, anxiety, smoking, alcohol abuse, substance use, and suicide have been reported among MSM as a result of chronic stress and disconnection from a range of social services and support mechanisms.