The yield and quality of a crop to be harvested depend in part on the quality of the seed that is planted. Seed quality is a relative term. It is based on a number of conditions, the more important of which are purity of the seed as expressed in terms of freedom from weed seed and other impurities, viability, freedom from seed-borne organisms and ability to resist infection by soil-borne organisms. Many factors and conditions affect seed quality, including crop sequence, type and frequency of cultivation, climate, time of harvesting, genetic constitution of the seed, variety of seed planted, storage, age and treatment of seed. The purpose of this bulletin is to discuss some of the factors which affect seed quality, to outline methods employed in a seed laboratory to determine seed quality and to illustrate how the results of seed laboratory tests may be applied in the planting of seeds.