Pharmacokinetics is in essence the study of the input/output relationships of the (human) body, which is considered as a system characterised by a density function of residence times. The input is the dosage rate, and the output is the concentration in the blood. The body transport function is first derived in a model-independent fashion, assuming linear kinetics. It is subsequently defined on the basis of a positive feedback loop of transport through the pulmonary and systemic circulation. Thus, the residence times distribution is based on transit times distributions and recirculation. The relevant dynamic systems parameters are cardiac output, extraction ratio, clearance, volume of distribution, mean transit time and mean residence time. In the case of drug absorption, mean absorption time and bioavailability are also important. In this review, the systems approach in pharmacokinetics is illustrated by clinical and computational experiments.