The present article is a review written at a level suitable for students and new workers to the field of techniques in common current use for the measurement of static and dynamic features of metabolism, especially nutritional metabolism. It covers the nature of radioactive and stable-isotope tracers, the means of measuring them, and the advantages and disadvantages of their use. The greater part of the review deals with methods for the measurement of pool sizes and metabolic processes, with the emphasis being on protein metabolism, a field the author knows best. The examples given are from a variety of sources, including the work of the author, but the principles underlying the techniques are universally applicable to all metabolic investigations using tracers.