Assessment of the HIV/AIDS dynamics requires estimation of total number of people living with HIV, including those who are diagnosed and who are not yet diagnosed, estimation of number of new infections and HIV death each year.The major obstacle of the assessment exercise is the estimation of the size of undiagnosed HIV positive population. Traditional methods are various form of statistical back calculation methods. These estimations are crucial for public health authorities to assess the most important risk factors for the HIV/AIDS dynamics, to plan for future HIV control measures and optimal allocation of resources.In this talk, I will show how mathematical models can be used to perform these estimations for China. It is based on collaborative research with the China Centrefor Disease Control and Intervention.