This chapter discusses mainly the literature published since 1981 and the recent developments in the use of stable isotopes for investigations of drug metabolism and disposition. The isotope cluster (isotope doublet, twin ion) technique has proved to be a powerful method for the detection and structural identification of drug metabolites in complex biological matrices. Although a useful adjunct to the stable isotope cluster technique, the administration of radiotracers in human research is limited on ethical grounds, particularly in pediatric and obstetric populations. Studies on the biotransformation of theophylline in premature newborns and on the placental transfer and fetal metabolism of this compound in humans relied exclusively on the use of theophylline as tracer. Although the use of deuterium in drug metabolism, involving the isotope cluster technique, may be complicated by isotope effects, both cost and convenience have led most workers to rely on this isotope. Recent examples include reports on arninopyrine, pencycuron, phencyclidine, fentanyl, and diethylstilbestrol. The use of stable-isotope-labeled compounds as internal standards for the quantitation of drugs and metabolites in biological fluids offers a unique combination of sensitivity and selectivity of detection for the pharmacokinetic studies. Stable isotope techniques offer a number of significant advantages over alternative methods for investigations of drug metabolism and disposition. Some of these have been highlighted in the chapter and are widely appreciated.