Unmet need for contraception has been a central indicator for monitoring the progress of family planning programs for 25 years. The purpose of this article is to provide a broad context for the more focused contributions that follow in this special issue. The validity and measurement of the concept of unmet need are discussed. We then present regional trends among married women since 1970. Major reductions in unmet need have been achieved, with the clear exception of sub-Saharan Africa. Less success can be claimed in addressing the needs of sexually active unmarried women, who contribute nearly 20 percent to overall unmet need in developing countries. Prominent reasons for unmet need in settings where contraceptive uptake is low include social resistance and insufficient information concerning methods. As contraceptive use increases, the importance of these reasons wanes, but concerns regarding side effects and health impact remain a barrier, and discontinued users now constitute a large proportion of those with unmet need. Drawing on these reasons, we outline measures to further reduce unmet need.