Vaccines are now widely regarded as an effective and cheap tool for improving health. The chapter provides a brief summary of the history of vaccination and its impacts on human health. It looks at the state of play today and at the reasons why progress on vaccine delivery and development has slowed, and outlines why rate of return analysis and calculations of economic impacts suggest policy makers should direct more resources toward vaccination. It reviews both cost-effectiveness analysis and cost-benefit analysis, indicates that a broader view of the long-term benefits of vaccination makes immunization programs much more worthwhile, in terms of their economic consequences, than has been thought in the past. The chapter details our study on immunization and cognitive development, which has, in turn, been linked to higher earnings. It estimates the rate of return to one of GAVI's prospective investments, showing that this return is quite high.