Neutron Activation Analysis (NAA) is the most accurate method for determination of tracer elements in biological materials. As many as 70 elements can be measured in therangeof10-6 to 1-12 g. important tracer elements in the human body can be estimated in health and disease. Total body water, nitrogen, calcium, chloride, potassium body cell mass. In vivo NAA can quantify human body composition in aging process, malnutrition, AIDS, cancer, and effects of weightlessness in astronauts. Various clinical applications of neutron activation analysis have been described.