This paper reviews 83 studies that measure the impact of curriculum-based sex and HIV education programs on sexual behavior and mediating factors among youth under 25 years anywhere in the world. Two thirds of the programs significantly improved one or more sexual behaviors. The evidence is strong that programs do not hasten or increase sexual behavior but, instead, some programs delay or decrease sexual behaviors or increase condom or contraceptive use. Effective curricula commonly incorporated 17 characteristics that describe the curricula development; the goals, objectives, and teaching strategies of the curricula themselves; and their implementation. Programs were effective across a wide variety of countries, cultures, and groups of youth. Replications of studies also indicate that programs remain effective when implemented by others in different communities, provided all the activities are implemented as intended in similar settings.