There has recently been growing interest in various atomic and nuclear techniques for the measurement of elements in the body. This has arisen through the realisation that (a) clinically-important amounts of toxic elements can be absorbed as a result of low-level environmental exposure, and (b) important information about the nutritional status of a patient can be obtained from measurements of major body elements. Where such information can be obtained by taking samples, a very wide range of analytical techniques is available, some capable of a sensitivity measured in parts-per-billion. Sampling is not possible, however, when the whole-body content (e.g. of nitrogen) is required, and is clinically undesirable when the element in question is concentrated in particular organs, for example as lead accumulates in the bones, and cadmium and many other toxic elements accumulate in the kidneys. It is in such cases that the various in vivo techniques are particularly important.