Interest is increasing worldwide in on-farm conservation as a component of a strategy to conserve crop genetic resources. This may require outside support to small-scale farmers in areas of crop diversity. This paper argues that crop diversity maintained by farming households results from the interplay between a demand and a supply for this diversity. Interventions to support on-farm conservation can be conceptualized by the way they influence these two factors. Demand interventions should increase the value of crop diversity for farmers or decrease the farm-level opportunity costs of maintaining it, while supply interventions should decrease the costs of accessing diversity. Español