Mesothelial lesions of the peritoneum may pose significant diagnostic problems. Accurate identification of these lesions is important because of their distinctive behavior and treatment requirements. This review discusses the clinical, pathological, and differential diagnosis of a variety of mesothelial lesions of the peritoneum, which includes mesothelial hyperplasia, multilocular peritoneal inclusion cysts (benign cystic mesothelioma), well-differentiated papillary mesotheliomas, and diffuse malignant mesothelioma, and the role of immunohistochemistry in the distinction from the more common primary peritoneal serous carcinomas. In addition, borderline serous tumors, psammocarcinomas, and solitary fibrous tumors of the peritoneum are briefly mentioned.