The term men who have sex with men (MSM) encompasses a diverse group of men of all ages, ethnicities and cultural backgrounds. Their sexual behaviour is equally diverse: while some MSM have exclusively male partners, others have partners of both genders or identify themselves as heterosexual but have occasional casual sexual encounters with men. MSM may be involved in the commercial gay scene, may visit cruising environments in public places or may be in monogamous sexual relationships. There were an estimated 415,600 MSM aged 15-44 in the UK in 2007. The estimated prevalence of HIV in this group was 5.2%. Of 60,050 people aware of their HIV-positive diagnosis in the UK, 26,050 (43%) were MSM. A further 9,000 MSM in the UK are estimated to be infected with HIV but unaware of their diagnosis.