Peritoneal dialysis prescription in children should be individualized--based not only on numerical targets (Kt/Vurea, Kcreat), but also on consideration of the peritoneal membrane, a dynamic dialysis membrane. In fact, the effective peritoneal surface area is at least a triple entity: an anatomic area, a contact area, and an exchange area. The anatomic area appears to be twice as large in infants as in adults if expressed per kilogram of body weight (BW), although the area is independent of age if expressed per square meter of body surface area (BSA). Therefore, scaling of the intraperitoneal fill volume (IPV) by BSA in square meters is necessary to avoid a low IPV/area ratio, which results in a functionally "hyperpermeable" peritoneal exchange. The contact area (the wetted membrane) is only a fraction of the anatomic area--that is, 30%-60% in humans (by computed tomography). Contact area depends on a variety of factors, such as posture and fill volume, that affect the degree of recruitment of membrane contact area. The exchange area is influenced by both the anatomic are and the contact area. However, it is mainly governed by the specific vascular area as determined by the peritoneal vascular perfusion and the capillaries available for exchange. Vascular area is dynamically affected by a variety of factors, such as the composition of the peritoneal dialysis fluid, the fill volume, and possible inflammatory agents.