Malnutrition can be defined as a lack of a sufficient quantity or quality of nutrients to maintain the body system at some definable level of functioning. It has been estimated that nearly half the children in the world today grow and develop under conditions of protein or protein-calorie malnutrition sufficient to impair their physical development. In the United States this level of malnutrition is rare. The major problem in the United States is probably that of hunger, defined as a psychological variable. It seems probable that the social and psychological causes and consequences of malnutrition and hunger are themselves conceptually independent. In this paper some of the factors involved in the relationship between malnutrition, learning, and behavior are critically reviewed. Most of the data are drawn from studies conducted in the developing countries of the world, where the conditions of malnutrition are significantly different from those in the United States. Therefore attention is given to some speculations about the impact of hunger on learning and behavior in this country. The review of the impact of malnutrition on learning and behavior concentrates on methodological and conceptual problems of studies reported in the literature. These problems make it impossible to assess the impact of malnutrition, as a separate entity, on human behavior. Malnutrition is socially regulated, and not random, either in the social system as a whole, or in segments of the social system, be they sub-classes or families. The conditions which lead to malnutrition are also conditions which affect the intellectual, social, and cognitive development of the child. (Author/JM)