A physiologically based model of lead kinetics in children and adults has been developed and tested. The premises on which the physiologically based model is founded are reviewed in this paper. Because 95% or more of the body burden of lead in adults is found in the bone, bone metabolism is central to the model. Bone volumes are expressed as functions of body weight. Bone formation and resorption rates are estimated from human studies of stable labeled calcium kinetics. Cortical and trabecular bone are modeled separately, with their surface-to-volume ratios taken into account. Standardized growth curves are used to relate body weight to age. Other model features such as organ volumes and physiologic functions are related to body weight based on measurements made in human subjects over a range of ages. Calibrations of the model to two human data sets are shown, and two applications to specific research questions are illustrated. A brief comparison of the structure of this model with that of the Leggett model, and a comparison of the output of this model with that of the integrated exposure uptake biokinetic model of the U.S. Environmental Protection Agency, are also included.