The peritoneum is defined as the serosal membrane that covers the peritoneal cavity. The main role of peritoneum is to fix and protect the intra-abdominal organs and to minimize the friction between intra-abdominal organs, including the abdominal wall, that is caused by intestinal peristalsis or respiratory movement. The peritoneum is composed of visceral and parietal peritoneum, and has different artery, vein, and nerve supplies. Under normal physiological state, 80% of the intra-abdominal fluid is absorbed from visceral peritoneum, so small molecular substances in the abdominal cavity are absorbed into the portal vein and then rapidly metabolized in the liver. The human peritoneal membrane is composed of two primary layers: the mesothelium, which is composed of a monolayer of mesothelial cell, and connective tissue layer under the mesothelium. These two layers are separated by the basement membrane. In mesothelial cells, various substances are produced and secreted, and substances such as proteoglycan and hyaluronan that decrease friction, and mediators such as cytokines and growth factors are also produced and secreted. Therefore, mesothelial cells play an important role in the physiological function of the peritoneum. The peritoneal microvessels are located within connective tissue layer and play a major role in the transport of small molecular substances during peritoneal dialysis treatment. In peritoneal dialysis, solutes are removed by diffusion, in which solutes move through the peritoneum due to differences in concentration, and convection, in which solutes move along with the movement of water. Diffusion is the main mechanism of solute removal. A three-pore theory can explain the transport of solutes and water through the peritoneum: transcellular pore, small pore, and large pore. The ability of a substance to move through the peritoneum is determined by the effective surface area and intrinsic permeability of the peritoneum, and the intrinsic permeability is related to the capillary permeability. The effective surface area of the peritoneum is determined by the number of perfused capillary vessels present in the visceral and parietal peritoneum, and the rate of movement of small- sized solutes such as glucose, urea, and creatinine reflects the effective surface area of the peritoneum. The water moves through the peritoneum consist of an ultrafiltration phenomenon in which water moves by the difference in osmotic pressure.