The science of agrobiodiversity has emerged during the last 10 years. We review here the most important aspects of biodiversity for conservation. One of the aims of agrobiodiversity research is to introduce or to re-introduce into present-day agriculture and horticulture more diversity from gene banks, botanical or zoological gardens, and other secondary sources of diversity. To enlarge the basis of agricultural and horticultural resources for human and animal nutrition, a sustainable use of these native and cultivated resources is necessary, including animal and plant genetic resources. The total number of botanical plant species cultivated as agricultural or horticultural crops is estimated at almost 7,000. However, only 30 major crop species "feed the world". Comparable numbers of animal species have been lost. The reduction in crop species and variety diversity, in particular, has led to the establishment of germplasm collections, so called gene banks, or ex situ collections. Six million plant accessions are conserved in gene banks worldwide. All these accessions belong to a very limited number of species. About half of them are advanced cultivars or breeders' lines, and only a third are landraces or old cultivars. Approximately 15% are wild relatives of crop species and weeds. Among other obvious gaps, minor crops and underutilized species are underrepresented in these collections, particularly primitive cultivars and wild relatives from the centers of origin, diversity, and cultivation. To date, only a third of all gene bank accessions have been fully characterized.