As in other areas of conservation, the crucial question concerning plant biodiversity isn't whether to conserve but how best to conserve diversity at the different levels of biological organization--the gene, the species and the community. Conservation biology is faced with several controversial issues, such as the dichotomy between the preservation of individual species versus a broader focus on the environment, the relative importance to give to endangered species, the design and management of reserves, and the drive for increasing agricultural productivity through plant improvement versus the drive to maintain traditional peasant varieties in cultivation. These and other issues are dealt with here by emphasizing and exploring the underlying scientific principles. The conservation of whole communities emerges as the paramount strategy for maintaining the evolutionary potential of plant life.