Underutilized plant species are defined as agricultural or non-timber forest species that are locally abundant in developing countries but globally rare. Scientific information about them is scant and their use is currently limited relative to their economic potential. Some are potentially high-value crops and they all contribute to agricultural biodiversity and the livelihood of the poor. Despite a growing body of scientific literature on underutilized species, to our knowledge, agricultural economics literature has contributed little to the understanding of how to commercialize these crops of plant products successfully. In this paper we first define what economic factors characterize underutilized plant species. Our classification of species is based on: 1) the relationship of the observed to the potential economic value of the species; 2) the presence or absence of an output market; and 3) the presence of market imperfections and 4) the presence of particular market failures. With this economic characterization, we exclude species for which developing markets is in or of itself irrelevant. We then identify three necessary conditions to the successful commercialization of underutilized plant species for the poor: demand expansion, increase efficiency of supply and supply control mechanism. The purpose of developing this simple conceptual framework is to provide a basis for the design of an empirical investigation of marketing solutions for underutilized plant species among the rural poor in developing economies.