Despite recent studies on improved seed varieties estimating the adoption rates of maize in 1998 at 40 percent (Manyong et al. 2000) and rice at 60 percent (Larsson 2005), true adoption rates appear to be unknown. This knowledge gap exists due to the ambiguity surrounding what constitutes improved varieties in Nigeria, because they become mixed with traditional varieties once they are adopted and start being traded in the informal seed sector. Given existing evidence and the above caveat, it may be concluded that the rate at which the newly improved varieties of rice, cowpea, and maize are spread among farmers in Nigeria is significantly lower than expected. This slow adoption rate can be attributed to constraints on both the supply and demand sides. Supply side constraints have often been analyzed at various stages, including seed production, distribution, and certification. On the other hand, the constraints on the demand side have not been sufficiently analyzed and there is little systematic understanding of such constraints. Seed demand in Sub-Saharan Africa (SSA) exhibits distinctive characteristics which differ from the demand in the rest of the world. Diversity in seed demand behavior across farmers is particularly prominent in SSA. This diversity is due to (1) the large number of farmers using seed relative to the aggregate quantity of seed demanded; (2) farmers' inability to overcome their specific agro ecological and socioeconomic environments; and (3) poorly integrated markets. One key goal in characterizing Nigerian farmers' seed demand is improving the public sector's ability to predict which of them are likely to adopt new and improved varieties in the foreseeable future given their agro ecological and socioeconomic situations. This review focuses primarily on the demand for improved seeds of rice, cowpea, and maize. The key research questions addressed in this review are: (1) how has seed demand in SSA been generally characterized, (2) how does such characterization apply to Nigeria, (3) what are the knowledge gaps that, when filled, will allow the public sector to better target the farmers who are likely to adopt improved seeds in Nigeria in the foreseeable future? To that end, this review first describes determinant factors and diversity patterns of farmers' seed demand observed in Eastern and Southern Africa and reported by Minot et al (2007). The study then reviews relevant studies in Nigeria to assess (1) whether any empirical studies on Nigeria report evidence that the same patterns apply to Nigeria; and 2) how such characteristics lead to the demand for specific forms of support from the public sector. This review also discusses some of the unique characteristics of rice, cowpea, and maize, as well as the unique characteristics of Nigerian socioeconomic conditions relevant to the seed sector, in order to highlight how such uniqueness relates to country- and crop-specific characteristics of seed demand in Nigeria. This review contributes to the literature in two ways. First, it brings together the main studies in Nigeria which analyze farmers� seed demand behaviors and identifies the key knowledge gaps that should be filled by future empirical studies in Nigeria. Second, the review provides further evidence from Nigeria on the general characteristics of SSA farmers' seed demand compiled by Minot et al (2007) in Eastern and Southern Africa. Reported evidence generally supports the proposition that farmers' demand for seed is highly diverse, and is affected significantly by agro-ecological and socio-economic conditions. Such evidence suggests the need for a more systematic understanding of farmers' demand to assist seed sector policy formulation.