Early intervention services have been introduced in a number of countries, but the evidence base to support them is limited. In particular there are very few economic evaluations, which are crucial if decision-makers are to have a better understanding of how scarce resources can be used appropriately. This paper discusses the different approaches used in economic evaluations and shows how these differ in the way in which outcomes are measured. The most useful forms of evaluation are cost-effectiveness and cost-utility analysis. We describe how the results of evaluations can be interpreted using incremental cost-effectiveness ratios and acceptability curves. Finally, the paper summarises some key evidence to date on early intervention services and economic evaluations currently being undertaken.