Genebanks conserve genetic recources ex situ and make them available to users. The nature of the resources conserved and how they are maintained depends on the objectives of the genebank, its responsabilities, policies and users. A genebank may have an institutional, national or international mandate and store material as seed, whole plants in the field or as tissue cultures, in vitro. It may be conserving genetic resources for use today and tomorrow, or may additionally have a more long-term insurance conservation mandate. Implications of policy, objectives and the material held, on the management and design of the genebank, are discussed later. First the paper explores the scientific and practical factors undelying the effectiveness of genebank operations