Female sex workers are at high risk for infection with HIV, and their clients may act as a bridging population by spreading HIV to the general population. Comprehensive HIV surveillance among sex workers includes surveillance of HIV infection, of sexually transmitted infections and of risk behavior. Surveillance of HIV infection among sex workers is critical for countries with low-level or concentrated HIV epidemics, and can help in monitoring the response to the HIV epidemic in countries with a generalized epidemic. Sex workers are a vulnerable population, and particular attention needs to be paid to human rights issues including consent, confidentiality and stigma. Collaborations with key players in the local sex work scene--sex workers themselves in the first place--and alliances with salient institutions and groups are key to the success of surveillance among sex workers. Surveillance activities should have a strong link to interventions targeted at sex workers. Surveillance for HIV infection among sex workers can be institution- or community-based. Institutional settings include screening programs for registered sex workers, of sexually transmitted diseases clinics, and re-education camps. Specific sources of bias need to be considered in different settings, and must be measured--through the collection of socio-demographic and behavioral data--to allow a correct interpretation of prevalence data and time trends. Community-based HIV infection surveillance can be conducted in a probability sample of the sex worker population, thereby reducing selection bias. This requires the mapping of sex workers' contact venues, and drawing a random sample from the resulting sampling frame.