The current state of quantitative analysis with the mass spectrometer in biochemistry and medicine is reviewed. The basic principles of mass spectrometry, the latter's combination with chromatography, the development of sensitive, exact, and in particular specific mass-spectrometric methods of detection, and the principle of dilution with stable isotopes are illustrated by examples. The most important fields of application are: the pharmacokinetics of drugs and active metabolites, investigation of metabolic pathways, supporting of medical diagnoses and enhancement of their specificity, and finally checking the quality of simpler quantitative processes in clinical chemistry.