The Optimus Study in China is a national study on prevalence and annual incidence of child sexual abuse (CSA) in China comprising epidemiological data (population survey) and official data (agency survey) that will take place between 2008 and 2011. The objectives of the population survey are to estimate prevalence and annual incidence of child victimization in China, including Hong Kong and 5 provinces of the Mainland China. A cohort of parents or guardians of children aged 0 - 17 and students aged 15 - 17 drawn from large representative samples from households and schools in Hong Kong and 5 Mainland provinces will be interviewed face-to-face by trained interviewers. The agency survey shall provide an annual estimate of incidence known to authorities and shall become a baseline for tracking change in terms of the CSA count and the structure of the response system. It shall also be a baseline for comparisons between regions and grounds for informing policies. The survey will cover agencies such as police departments, hospitals and non-governmental organizations (NGOs) offering services to victims and perpetrators of CSA. The present research aims at bridging existing research and information gaps with regard to the incidence and prevalence of child victimization in China. This study will have significant policy implications. Information obtained from the study will facilitate evidence-based planning and policy-making.