Gender analysis is central to a deeper understanding of HIV transmission, argues an Institute of Development Studies BRIDGE report, given that sexual behaviour is a crucial factor in spreading the disease. It is now understood that current global levels of HIV and AIDS are far higher than previously thought. Analysis of existing data also reveals that women and children are affected more directly than men and are affected differently, especially in Sub Saharan Africa. There is evidence that heterosexual transmission is on the increase meaning that more women will become infected. Yet, how reliable is current data? What improvements can be made to data collection systems? How will better data help in the battle to reduce transmission of this deadly virus?