These lectures were prepared by the authors for Seminars to be held on June 6, 1983, in St.Louis, Missouri, under the spon- sorship of the Radiopharmaceutical Science Council of the So- ciety of Nuclear Medicine . All manuscript? were refereed . Tracer kinetics and the modeling of physiological and bio- chemical processes in vivo are the focus of a contemporary di- rection in biochemical research. Recent advances in instrumen- tation (especially positron emission tomography and digital autoradiography) and parallel developments in the production of short-lived radionuclides and rapid synthetic chemistry to pre- pare tracers that probe metabolism, flow, receptor-ligand kinetics, etc., are responsible for new scientific frontiers. These developments, coupled with biomathematics and computer science, make it possible to quantitatively evaluate tracer kinetic models in animals and man. (The choice of animal models in radiotracer design and tracer kinetics is the subject of a book edited by R.M.Lambrecht and W.C.Eckleman in press at Sprin- ger Verlag.) Tracer kinetics and physiological modeling is truly multi- disciplinary, as evidenced by the intellectual diversity and international representation observable in the list of contri- butors . The lectures outline and attempt to show the transition from the theoretical description to the practical application of modeling for understanding normal and pathological processes .