This primer endorses the importance of market segmentation to overall market growth. The primer serves the following purposes: (1) introduce the purpose, theory, process, and approaches for segmenting markets to increase family planning (FP) market efficiency and equity, (2) make the case for using market segmentation to expand access to and affordability of modern contraception, (3) provide all stakeholders—policymakers, public and for-profit providers, manufacturers, distributors, donors, program implementers and contractors, and NGOs—with a common set definitions and sources, (4) collect in a single place reference materials, websites, and data to facilitate use of market segmentation approaches in diverse country settings.