The overwhelming majority of HIV/AIDS patients live in the developing world. The rate of HIV infection in China has been consistently and steadily increasing over the past few years. Commercial sex workers and their clients are at highest risk for HIV infection and transmission. Yunnan Province is reported to be home to about half of China's HIV infected population. Honghe Prefecture accounts for 14% of the total reported HIV infections in Yunnan Province. Even though the incidence of HIV is important in tracking the HIV epidemic, there are little data on HIV incidence among FSWs in China. Estimates of HIV incidence are essential for optimally monitoring the HIV epidemic, knowing where to focus prevention efforts, and evaluating the success of vaccines and other prevention strategies. The purpose of this study is to estimate the incidence of and risk factors for HIV infection among FSWs and miners in Honghe Prefecture, Yunnan Province, China. In addition, this study will identify and describe the different HIV clades currently circulating among FSWs and miners. Four cross-sectional surveys will be conducted: three among FSWs and one among miners. In addition, a 12-month cohort study will be conducted among a subgroup of FSWs enrolled in the baseline cross-sectional survey in Kaiyuan City, Honghe Prefecture, Yunnan Province, China.