Professor Hart's research interests include sexual risk behaviour and the prevention of HIV and STIs, combining structural, behavioural, and biomedical approaches to prevention and the opportunities afforded by new prevention technologies. He has worked with a wide range of populations at risk of HIV and STIs, nationally and internationally. This includes men who have sex with men, young people, sex workers, and vulnerable groups in developed and developing countries.

His major contributions are to health policy and promotion, particularly in HIV and sexual health. Two significant examples are: needle exchange for injecting drug users (IDU); HIV prevention in men who have sex with men (MSM).

After needs assessment which found high levels of sharing of drug injecting equipment amongst IDU he was instrumental in setting up, and then evaluating, the first dedicated needle-exchange in the UK and the roll-out of this harm reduction strategy in England and Wales using combined behavioural and biomarker data. This work has been cited internationally, helped with the adoption of harm minimisation globally, and contributed to reduced parenteral transmission of HIV throughout the world.

His research on men who have sex with men is widely cited and has informed HIV prevention strategies in the UK and internationally. He undertook the first community-based survey of prevalent HIV using oral fluid antibody tests and the first UK evaluation of peer-education for risk reduction in MSM. He led work which first described in Scotland increased risk-behaviour after the introduction of HIV therapy in the mid-1990s, and subsequently specified the undiagnosed fraction of HIV in community-recruited MSM across the UK. This work also demonstrated high levels of sexual risk taking in HIV positive men. These studies have informed the development of HIV prevention in MSM in the UK and internationally and been vital for government and NGO prevention efforts.