This article aims to build a picture of HIV epidemiology in Europe by combining existing surveillance data to mathematical modelling to achieve observations closer to the dynamic reality of HIV infections across different parts of Europe. In the European Union (EU), where it is estimated that 30% of HIV-infected persons have not been diagnosed, the number of new HIV diagnoses has risen in recent years. However, trends must be interpreted with some caution around the differences and variations in surveillance systems and testing rates among affected populations and regions. By introducing mathematical models, we can build an overall picture from the pieces of information available. We present a mathematical model of the course of infection and the effect of ART which has been developed to fit as closely as possible to observed data from HIV cohorts. The preliminary estimates for the entire WHO European Region are that around 2.3 million people were living with HIV in Europe at the end of 2006, of whom around 50% have not been diagnosed. The model can also be used to assess the potential impact of earlier diagnoses. Observations show how a combination of surveillance data and modelling allows an estimation of the current state of the epidemic in Europe, though further developments in both areas are needed.