The subject for a volume on the fat-soluble vitamins needs no justification considering the importance of this group of nutrients and the rate of expan- sion of our knowledge of its role in cell biology, genetics, and disease. The level of our understanding has clearly moved from knowing what fat- soluble vitamins do to how they perform their functions. Hand in hand with a knowledge of their molecular mechanisms of action is the recognition that vitamins are used sparingly, and regeneration processes operate in certain cases to recycle vitamins from their metabolites. We have divided the...