Data on the fate of sex steroids in the human organism, such as absorption, distribution, total clearance, and elimination routes, are necessary to understand and predict drug action. However, except for a few exceptions, such data are sparsely available. It is necessary to apply several approaches (descriptive, mathematic, and analytic) to describe variability and dose-response correlations, all of which are important for individualized treatment, and therefore of therapeutic relevance. In addition, such data are now unavoidable requirements for the evaluation of safety, efficacy, and quality performed by the regulatory authorities. Therefore human kinetic information is an integral part of the knowledge that forms the basis for any treatment with sex steroids.