Human papillomavirus is one of the most common sexually transmittable infections. An infection with a hr HPV type can persist and cause the development of malignancies on the anogenital site and in the head and neck region. To strongly reduce the transmission of HPV and development of (cervical) cancer, prophylactic, bivalent HPV vaccination was introduced into the Dutch NIP in 2009 as a girls-only vaccine, preventing the most oncogenic HPV types 16 and 18. The current thesis describes monitoring of the routine HPV vaccination program within the Netherlands using intermediate endpoints, given the large gap between occurrence of HPV infection and development of cancer. Serological measurements are important in vaccination research and evaluation, since they can provide information on the responsiveness to a vaccine. Therefore, in chapter 2, the populationbased changes in seroprevalence of unvaccinated individuals were evaluated over a ten-year time period, during which HPV vaccination was implemented in the Netherlands. IgG antibody levels to seven hr HPV types as induced by natural infection showed that seroprevalence increased over the past decade among unvaccinated women, while it was stable among men. A lower seroprevalence among young women or herd effects among men, which may follow from the recent introduction of the HPV vaccination program among teenage girls in the Netherlands, was not yet observed. In chapter 3, the focus was on vaccine derived immune responses. Antibody levels remained high up till nine years past vaccination (three doses), both for vaccine types and to some extent for cross protective types. Immune responses from vaccinated women who presented with an HPV infection were compared to immune responses from women without infection, but the difference was not significant in the year before infection. This indicates that an immune correlate of protection, i.e. a threshold that should be reached in order to be protected, cannot be easily determined. This was also described in chapter 4, ...