This chapter reviews the possible mechanisms of desiccation tolerance in seeds. It discusses some of the anatomical, biochemical, and biophysical events associated with the onset and loss of desiccation tolerance in orthodox seeds and compare these with developmental aspects of recalcitrant seeds. If cells can survive the immediate stress imposed on the structure of organelles and macromolecules when water is removed, they are often termed desiccation tolerant. For both recalcitrant and orthodox seeds, the relative level of desiccation tolerance changes throughout development so that embryos become more tolerant as they mature and less tolerant as they germinate. For many seeds, the timing of maximum dry matter accumulation and the acquisition of maximum desiccation tolerance of the embryonic axis are within days. Changes in carbohydrates also occur during seed and pollen development and germination. The plant growth regulator abscisic acid appears to play an important role in the development of desiccation tolerance.