Mature desiccation-tolerant (orthodox) seeds will retain viability for varying periods depending upon the species and the storage conditions. A number of distinct but interacting determinants are known to influence seed vigor and viability. These are genetic factors; preharvest and maturational effects; mechanical factors; the storage environment; intrinsic factors; and pathological factors. In contrast to the relatively consistent and logical information that has accumulated on free radical mechanisms operating in programmed senescence, the picture that has emerged on seed aging is controversial and disjointed. Recalcitrant seeds are shed at high water contents and are intolerant of dehydration and often also of chilling. During wet storage, when axis water content is maintained at the level characterizing the newly shed seeds, the ultrastructural situation is indicative of the ongoing germinative events, culminating in cell division that accompanies hypocotyl elongation and root growth. It is generally agreed that free radical mechanisms are important in the loss of viability of stored orthodox seeds.