The differences in access to and utilisation of health services, and differential benefits from health services, discussed in the previous chapter, may affect inequalities in population health in both positive and negative ways. It is difficult to obtain clear evidence as to the impact of health service utilisation on population health and health inequalities (and even more so when looking at the impact of health systems). Much of the evidence on health service utilisation and its effects refers to specific services or diseases and lacks an analysis by social groups. We are analysing a complex system that is difficult to capture in one comprehensive model, let alone in one empirical study. This chapter explores ways in which differences in health service utilisation and benefits between people with different socioeconomic backgrounds can be aggregated to the population level. We used the term ‘aggregated’ to denote the transformation from micro-level outcomes to population-level outcomes in terms of inequalities (Sect. 7.2). This also requires a definition of ‘the population’ (Sect. 7.3). We then discuss some potential solutions to the problem of transforming individual outcomes to population level (Sects. 7.4–7.8). It will be clear from this chapter that simple aggregation does not solve the problem.