There is a large and rapidly growing body of literature on the importance of mag nesium in biochemical and physiological processes. There is also much evidence that magnesium deficiency, alone and in combination with agents that interfere with its utilization, is associated with functional and structural abnormalities of mem branes, cells, organs, and systems. The manifestations of the changes caused by magnesium deficiency depend upon its extent and duration and on variable factors. Among the conditions that increase the risk of magnesium deficiency are (1) meta bolic factors that affect the absorption, distribution, and excretion of this mineral; (2) disease and therapy; (3) physiologic states that increase requirements for nutrients; and (4) nutritional imbalances. Excesses of nutrients that interfere with the absorption or increase the excretion of magnesium-such as fat, phosphate, sugar, and vitamin D-can contribute to long-lasting relative magnesium deficiency. All have been implicated in several of the diseases considered in this book. Whether their influence on the need for magnesium is a common denominator remains to be investigated further.