The dialytic treatment of end-stage renal disease (ESRD) patients is based on control of solute concentrations and management of fluid volume. The application of the principal of conservation of mass, or mass balance, is fundamental to the study of such treatment and can be extended to chronic kidney disease (CKD) in general. This review discusses the development and use of mass conservation and transport concepts, incorporated into mathematical models. These concepts, which can be applied to a wide range of solutes of interest, represent a powerful tool for quantitatively guided studies of dialysis issues currently and into the future. Incorporating these quantitative concepts in future investigations is key to achieving positive control of known solutes, and in the analysis of such studies; to relate future research to known results of prior studies; and to help in the understanding of the obligatory physiological perturbations that result from dialysis therapy.