In vivo neutron activation has opened a new era of research on the elemental composition of the human body. The techniques currently employed vary widely with respect to sources, moderators, site of activation and detection systems. Sources include cyclotrons, neutron generators and radioactive elements. Both partial body and total body neutron activation analysis (PBNAA and TBNAA, respectively) are commonly used to determine calcium levels. This review examines various aspects of the two techniques for delayed gamma neutron activation and the factors which affect the sensitivity of the measurements, uniformity of the neutron flux density, and the use of moderators. Portions of the body selected for partial body activation analysis are the hand, the arm and the trunk. Such measurement may be particularly useful for studying patients with diseases that affect various parts of the skeleton differently. However, to date, analyses based on TBNAA and PBNAA do not appear to favour one technique over the other. Comparison of the two techniques has to take into account a variety of factors: dose to the patient, cost reliability, availability of source, nature and cost of complementary facilities required, and the degree of expertise needed by the operating personnel. Neutron activation studies of body calcium have provided data useful for the diagnosis and management of a variety of metabolic disorders. Measurement of sodium, chlorine and nitrogen also appear to be useful clinically. A variety of clinical applications are discussed in this review. A recent development is prompt gamma neutron activation analysis, which can be used for the in vivo determination of cadmium in liver and kidney. Total body nitrogen (measured by prompt gamma neutron activation) and potassium measurements serve as indices of protein and muscle mass content, and hence are useful in assessing the roles of diet and nutrition in these body components.