In 2007, an estimated 33.2 million people in the world were living with HIV, and despite twenty years of prevention programmes, an estimated 2.5 million new infections occurred in that year. Underpinning the shortcoming in the prevention response is the inadequate use of evidence to inform the response. The result has been largely ineffective prevention interventions, with non-optimal use of available resources and the loss of early opportunities to address the unique factors driving infection in the populations most at risk within the country. The overall objective of this study is: 'to contribute to the ongoing efforts to understand the epidemic and response in Kenya and thus help the country improve the scope (doing the right kind of activities), relevance (with the right populations) and comprehensiveness (reaching all members of target populations) with HIV prevention efforts', with an ultimate goal of helping Kenya make more effective HIV/AIDS-related decisions. The report then assesses the relevance, comprehensiveness and cost of major HIV responses in relation to the epidemiological analysis and policy environment, and draws some conclusions about the state of the epidemic and about whether the prevention responses (and the resources allocated to them) are congruent with the evidence on where resources should best be directed. This study describes the epidemiology of HIV in Kenya over time: trends in HIV prevalence and incidence, magnitude and current phase of the epidemic, the main transmission pathways for new infections; and the heterogeneity of the HIV epidemic (by sex, geography, age group, and risk behaviors).