Genebanks around the world hold collections of the genetic resources of crop plants for long-term conservation and for ease of access by plant breeders, researchers and other users. Perceiving that the large size of some collections could deter use, Frankel (1984) proposed that a limited or "core collection" could be established from an existing collection. With minimum similarity between its entries the core collection is of limited size and chosen to represent the genetic diversity of a large collection, a crop, a wild species or group of species. It does not replace the existing collection or material from which it is obtained. Core collections have become accepted as efficient tools for improving conservation and use of collections. The Global Plan of Action for the Conservation and Sustainable Utilization of Plant Genetic Resources for Food and Agriculture (FAO 1996) recommends core collection development as one of the activities needed to improve use of plant genetic resources. This technical bulletin sets out the procedures that can be used to establish, manage and use a core collection, drawing on the accumulated experience so far.