Unmet need for family planning is defined using survey data as the percentage of women who do not want to become pregnant but are not using contraception. Though the concept seems straightforward, the calculation is extraordinarily complex and has changed over time. To address these issues, DHS revised the definition of unmet need in 2012. Using the revised definition:(1)Unmet need is calculated the same way in all DHS surveys.(2)Unmet need is calculated the same way in DHS and the Multiple Indicator Cluster Surveys (MICS).(3)Unmet need can be used to reliably track trends over time and compare estimates across countries. This paper describes the rationale, process, and results of revising the definition of unmet need, and compares estimates of unmet need using the original and revised definitions for all DHS surveys from 1990-2010. The study also estimates unmet need for spacing and limiting, unmet need by background characteristics, total demand for family planning and the proportion of demand satisfied, and unmet need for sexually active unmarried women, all using the revised definition.