Mesothelial cells refer to the cells that make up the mesothelium, which consists of a layer of cells located above the contact surface where the pleura or peritoneum is attached to the organs. The main function of the mesothelium is to lubricate so that organ to organ, organ to pleura, and organ to peritoneum are well protected from mutual abrasion and damage. The lubrication and protection provided by the mesothelium is accomplished by substances secreted by mesothelial cells. The clinical pathological significance of mesothelial cells is that mesothelial cells can form mesothelioma, a tumor originating in the pleura and peritoneum, which can be classified as benign or malignant. Since mesothelioma occurs in the pleura and peritoneum, mesothelial cells can be present in large numbers in pleural and ascitic fluid. Mesothelial cells are significant in the differentiation of benign and malignant pleural and ascitic fluid, and it is usually believed that the occurrence of mesothelioma is related to environmental factors such as exposure to asbestos.