Kinetic models have been used in both hemodialysis (HD) and peritoneal dialysis (PD) therapies. Because many different theoretical models are available, users should choose a model that suits the purpose of their studies. In general, simple models are useful for clinical investigations and research; rigorous models may be available with the use of computers. Several commercial software packages with rigorous models specifically designed for PD are also useful for analyzing patient data and for constructing prescriptions. Although the overall mass transfer area coefficient (MTAC) is a parameter for diffusion transport, it may become a useful tool for predicting peritoneal ultrafiltration by defining a new "index for peritoneal diffusive selectivity" (IPDS), the ratio of the MTACs for urea and creatinine. Kinetic models are especially useful in treatments with relatively complex prescriptions such as combined PD and HD ("complementary") therapy, sample calculations of which are discussed.