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The signing of the Convention on Biological Diversity at the ‘Earth Summit’ in 1992, its ratification and its subsequent entering into force have highlighted the need for an approach to biodiversity conservation that employs both ex situ and in situ techniques in a complementary manner. Though much research has focused on ex situ techniques, less progress has been made in developing methodologies for the conservation of genetic diversity in situ. The definition of in situ conservation used in the Convention on Biological Diversity encompasses two distinct processes: conservation of wild species in genetic reserves and of crops on-farm. Of these two, the latter, where the genetic diversity of crop land races is conserved within traditional farming systems, has been the less studied and remains less well understood. While there are still relatively few practical examples of on-farm genetic resources conservation, most genetic diversity of immediate and potential use to plant breeders is found among land races, and there is evidence that it is being rapidly eroded. This paper attempts to set on-farm conservation within the context of plant genetic resource conservation as a whole, to introduces a possible generalised model for the conservation of genetic diversity on-farm and to promote debate around the science of on-farm conservation.