As HIV epidemics among gay and other men who have sex with men (GMSM) persist worldwide, the importance of novel prevention approaches is recognised. Evidence that antiretroviral therapy (ART) can decrease the likelihood of infection is informing emerging HIV prevention approaches, encompassing early initiation of treatment as prevention by people living with HIV and use of antiretroviral drugs as pre-exposure prophylaxis for people presumed to be uninfected. Despite widespread excitement, robust evidence of the beneficial effects of ART-based HIV prevention for GMSM remains limited. Also, theoretical models project widely varying effects of ART-based prevention on the future course of HIV epidemics among GMSM, drawing attention to the possible moderating role of differences in the achievements of local HIV responses and the critical importance of sustained protective sexual practices into the future. Ecological analyses and simulations of ongoing epidemics in major gay communities illustrate that the preventive effects of ART in many settings are being offset by increased sexual risk-taking, as reflected in stable or increasing HIV infection rates. Also, the effects of scaling up HIV testing and treatment among GMSM in settings that are often considered prime examples of the success of ART-based prevention may be levelling as 'scope for improvement' diminishes. ART-based approaches further extend the HIV prevention toolkit and substantially increase people's options to protect themselves and others. The future impact of ART-based prevention on HIV epidemics among GMSM ultimately depends on whether heralded responses offset, attenuate or compound the ongoing social and behavioural changes that drive increased sexual risk.