The concept of “unmet need for contraception”, has been used in the international population field since the 1960s. The concept was developed from the first family planning and fertility surveys conducted in developing countries, which found a disconnect between women’s knowledge, attitudes, and practices (KAP) about contraception. This gap between what the respondents knew, their fertility preferences, and behaviours to achieve their stated preferences, became known first as the “KAP-Gap” and was used as a strong rationale for investment in family planning programs. The subsequent development of the unmet need concept has been supported by the availability of datasets from over 75 countries collected by the Demographic and Health Surveys (DHS) program. In this note, the strengths and weaknesses of the unmet need indicator, the differences between demand and supply factors for unmet need, differences between unmet need and the intention to use contraception are summarised, and the relevance of investing in family planning programs clarified.