Studies described in this thesis underline the need to enhance HIV testing in the Netherlands, since a large proportion of HIV-infected persons remain unaware of their infection. In particular HIV testing among ethnic minority populations and commercial sex workers should be scaled up, especially in settings outside the capital. Moreover, to interrupt ongoing HIV transmission, it can be argued to implement notification of HIV. Ethnic minorities originating from Surinam, the Netherlands Antilles and sub-Saharan Africa are at high risk for STI, including HIV, and coinfections; prevention needs to be enhanced. Commercial sex workers, especially transgender and drug using sex workers are at high risk for HIV and sexual risk behaviour. In addition, monitoring antimicrobial resistance to gonococci is of high importance among sex workers and also other risk populations such as men who have sex with men. Given social, demographic and migratory trends, the population at risk for sexually transmitted infections will continue to grow, worldwide but also in the Netherlands. The future course of a concentrated HIV epidemic, such as in the Netherlands, depends on the frequency and nature of (sexual) contacts of members of core groups and the rest of the population. Therefore, continuing surveillance of HIV, STI and sexual and other risk behaviour of high risk populations is of major importance.