Farmers use various seed systems to obtain seeds for their next crop. The diversity of seeds determines, together with other aspects of the farming system, the biodiversity in farmed areas. Conservation strategies for crop genetic resources distinguish ex situ, on-farm and in situ components. The on-farm management of such resources is highly influenced by the seed systems that farmers use. Next to primary and secondary centres of diversity, this chapter introduces a tertiary source of diversity, based on the creation of new diversity through modern breeding. The impact of different seed systems on the management of crop genetic diversity is also analysed.