Prevalence and incidence are the two most important indicators of the state of an epidemic. The most common way in which incidence is measured is by follow-up of an initially uninfected cohort. For infections with a relatively short duration, another method for estimating incidence is available using a cross-sectional survey. Unfortunately, HIV has a long asymptomatic phase before the onset of immune failure and AIDS. In this article a way to estimate HIV incidence using biomarkers in cross-sectional surveys is described and the challenges of this approach are discussed.