The range of potential effects and costs of interventions to expand the coverage of immunisation programs in developing countries was reviewed for the first time and demonstrated that increasing coverage is achievable. With the exception of a mass campaign, all interventions were reported to increase the proportion of fully vaccinated children. The findings are of particular value for policy debates about the introduction of new vaccines versus the need to vaccinate as many children as possible with the current ones. The review highlighted the need for: cost analyses to be undertaken alongside evaluations of interventions; and an improvement in the methodological quality of studies.