The peritoneal membrane has the surface area similar to the body surface area. It consists of mesothelial cells, interstitium, connective tissue fibers, blood vessels, and lymphatics. Solutes of various sizes traverse the peritoneal membrane through at least three various pores: 'large' pores located in the venular interendothelial gaps, small 'paracellular' pores, and ultrasmall, 'transcellular' pores or aquaporins localized in peritoneal capillaries and mesothelial cells. High molecular weight solutes are mass-transfer limited; thus, their clearances do not increase significantly with high dialysate flow. Clearances of small molecular weight solutes are dialysate flow limited. Ultrafiltration is proportional to the hydrostatic and osmotic transmembrane pressures. The peritoneum offers greater resistance to accompanying solutes than to water (solute sieving), so that the concentration of solutes in the ultrafiltrate is less than in plasma water. Sodium sieving leads to hypertension, which is frequently observed in patients treated with short-dwell or continuous flow peritoneal dialysis. Peritoneal equilibration test is the most commonly used test to characterize peritoneal function and select the most suitable dialysis technique for a patient. Long-term peritoneal dialysis is associated with progressive loss of ultrafiltration capability due to structural and functional alterations in the membrane mostly as the consequence of exposure to glucose degradation products or advanced glycation end products generated during the sterilization process.