The use of NaI(Tl) scintillation spectroscopy has been extended successfully to the appraisal of the gamma radiation emitted by radioelements present in thc human body, both in its normal state and in the presence of contamination levels well below those accepted as maximum permissible under industrial exposure. Design criteria to be met by the detector and the shield of an instrument adequate for these purposes are discussed from theoretical and practical points of view. Special attention has been given to the analysis of the background spectrum as it is affected both by the natural radioactivity of the apparatus and its surroundings, and by the action of cosmic-rays ionizing particles and showers. General and special nmeasuring techniques, based on spectral analysis, are described in detail as they concern the localization, identification, and quantitation of various radioelements either distributed throughout the whole body or localized in some of its organs. An outline is given of a coincidence summing technique capable of measuring, without recourse to chemical procedures about 10/sup -11/ c of either the U/sup 238/ or Th/sup 232/ chain in equilibrium in thc presence of potassium gamma-ray activities higher by three orders of magnitude; possible uses of this technique are brieflymore » mentioned. (auth) « less