In the present paper, a survey of methods applied to the interpretation and evaluation of tracer kinetic data is given. For their mathematical description, both compartmental and non-compartmental models, such as the modified model of Sprinson and Rittenberg, the San Pietro-Rittenberg model, two models of the albumin metabolism and a 10-pool model of the N and protein metabolism were used. By means of single or multiple pulse, infusion and priming techniques, the N and protein metabolism in various metabolic states (e.g. healthy man, pathological and stress conditions, therapeutic treatments) were studied.