The genetic improvement of food staple crops cultivated by small-scale farmers is a well-established route to increasing agricultural productivity and improving rural livelihoods. But in developing countries where seed markets are commercially active or advancing in that direction, undue emphasis in both policy and research is often placed on the adoption of improved cultivars rather than varietal turnover, or the replacement of an already improved variety with a more recently released improved variety. Strong and consistent rates of varietal turnover contribute to sustaining yield gains over time, protecting those gains from both biotic and abiotic stresses, increasing the sustainability of intensive cropping systems, and improving the quality of the commodity itself for storage, processing, and consumption. This paper explores the importance of varietal turnover in advanced and transitional seed systems for food staples in South Asia and Africa south of the Sahara. We first review the measurement of varietal turnover over spatial and temporal dimensions before examining evidence on policies designed to accelerate varietal turnover rates. We then suggest a sequence of regulatory reforms and public investments designed to accelerate varietal turnover while drawing attention to the economic trade-offs, unintended consequences, and operational challenges of such reforms and investments.