It has long been recognized that anemia leads to weakness and malaise. While there are many causes of anemia, by far the most common is iron deficiency. Prevalence rates range from 10 to 20 per cent for some populations in industrialized countries to 30 to 40 per cent or more in some developing countries. What has not been appreciated until recently is the extent to which iron deficiency, even when not severe enough to cause anemia, can be associated with a number of specific functional consequences. These include adverse effects on immunity and resistance to infection, on learning and behavior, physical capacity and work output, and possibly temperature regulation. Most of these effects are rapidly and completely reversed by administration of iron. The causes of iron deficiency are multiple and complex. It is not a simple matter of dietary iron intake because other substances in the diet influence iron absorption. Moreover, a variety of pathological conditions, such as menorrhagia, hookworm, malaria, and schistosomiasis, contribute to iron deficiency. The functional consequences of iron deficiency should be of concern to industrialized as well as developing countries, and preventive measures are mandatory. The United Nations University has established a network to collect evidence of the effects of iron deficiency and is cooperating with the International Anemia Consultative Group to stimulate research on practical means of preventing iron deficiency in all populations. These include fortification of staple foods, consumption of foods with more available iron, reduced consumption or altered timing of ingestion of foods that reduce iron absorption, and the prevention of blood loss. The paper will attempt to provide a summary and analysis of currently available information on the nature and consequences of iron deficiency in human populations and its prevention.