Pharmacokinetic models provide novel approaches in the environmental arena for evaluating toxicological problems. For example, pharmacokinetic parameters play an important and even determinant role in risk assessment and policy making. The present contribution reviews the most basic pharmacokinetic models, their development, and their applications in toxicology. The theory, principles, and data requirements are critically discussed. The fundamental differences among various pharmacokinetic models are compared and contrasted. The techniques and philosophy for model validation are discussed and illustrated.Key words: Pharmacokinetics, toxicology, modeling, philosophy, statistics, validation.