Nutrition is a critical determinant of immunocompetence and risk of illness and death largely due to infectious disease. It is now established that undernourished individuals have impaired immune responses. The most consistent abnormalities are seen in cell-mediated immunity, complement system, phagocytes, mucosal secretory antibody response and antibody affinity. These changes, together with other handicapping factors observed in underprivileged societies, lead to more infections. It is now recognized that deficiencies of single nutrients also impair immune responses. The best studied are zinc, iron, vitamin B-6, vitamin A, copper and selenium. If malnutrition occurs during fetal life, as epitomized by small-for-gestational age infants, the effects on cell-mediated immunity are very significant and long lasting. There is much recent evidence to suggest that at the other end of the age spectrum, namely old age, nutrition plays an important role in maintenance of optimum immunity. Based on these data, several studies have documented the critical importance of nutrition in resistance to a variety of infectious challenges, including Salmonella, Listeria and coxsackie B. Similarly, in vitro and in vivo responses to tumor cells are modulated by nutrition. These interactions of nutrition and immunity have several practical applications, including resistance to infections and tumors and the development of designer formulas that might help reduce the occurrence of opportunistic infections in immunocompromised hosts.