This paper explores the economic casefor investments in youth in developing countries. Thecurrent cohort of youth is the largest cohort ever. Theeconomic, social, and demographic context of their lives hasundergone enormous change, thus requiring a rethinking andre-evaluation of the range of investments in youth. Thisreappraisal must incorporate a number of critical featuresincluding recognition of the wide range of youthinvestments, the considerable lag in effects, and thelikelihood that youth investments in one area affectinvestments and behavior in other areas. The paper examinesforty-one investments in the following broad categories:formal schooling; civilian and military training, work;reproductive health; school-based health; other health; andcommunity and other. The paper develops a life-cycleapproach using cost-benefit analysis to calculate theeconomic returns to investments in youth. However, theinformation necessary to apply the methodology is sufficientfor only a few investments in a few countries. Moreover,even for these cases, the estimated economic returns varywidely depending on the assumptions used. Despite theselimitations, the available evidence suggests that some typesof investments in youth, e.g., investments in formalschooling, adult basic education and literacy, some types ofschool-based health investments (e.g., micronutrientsupplements and, under certain circumstances, reproductivehealth programs), and measures designed to reduce theconsumption of tobacco (e.g., increases in the tobacco tax),yield economic returns that are at least as high as arethose for many investments in other sectors. The lack ofreliable information on the effects of many investments inyouth is the most important information gap and the areameriting the highest priority for future research.