In pharmacokinetic modelling the body is represented as a set of compartments. The characteristics of these compartments are defined either by fitting predetermined mathematical equations to the data ('data-based compartments') or by defining compartments based on the actual biological structure of the animal ('physiologically based compartments'). Physiological models of chemical disposition are developed using these physiologically based compartments. These models then consist of sets of organs or types of tissue compartments whose characteristics are based as far as possible on the anatomy and physiology of the test species. Individual organs or types of tissue are defined with respect to their blood flow, volume, kinetic constants for metabolism, storage capacity for the compound involved, protein binding and other relevant characteristics. Linking these compartments together in a proper anatomical arrangement yields the physiological model for compound disposition. This paper provides an overview of the basics for constructing physiological models for organic compounds, focusing on the structure of individual compartments in these models and the data required for model development. Some past applications of physiological models are reviewed and speculation offered on future developments in this field.