This essay considers selected peritoneal lesions many of which were the subject of studies coauthored by Dr Robert E. Scully. His article on multilocular peritoneal inclusion cysts has largely led to these lesions being considered non-neoplastic, eschewing the term cystic mesothelioma. These cysts are often associated with reactive mural mesothelial proliferations that can potentially lead to a misdiagnosis of mesothelioma. Clinical findings, such as a common association with endometriosis or prior operations, can prompt consideration of a reactive lesion. Mesothelial hyperplasia may be difficult to distinguish, when florid, from mesothelioma but a variety of gross and microscopic features will aid their recognition. Nodular peritoneal aggregates of histiocytes (sometimes admixed with mesothelial cells) may occasionally be a striking finding that can be misdiagnosed as a metastasis if the patient has a known neoplasm. Appreciation of their bland nuclear features and histiocytic nature, confirmed by immunohistochemical markers, facilitate the diagnosis. Various forms of peritonitis are briefly considered including sclerosing peritonitis, a process sometimes associated with luteinized thecomas (thecomatosis) of the ovaries, an entity first appreciated by Dr Scully. Mesotheliomas are briefly reviewed emphasizing the caution that should be used in applying the designation “well-differentiated papillary mesothelioma.” Many interpret the latter as benign, but multifocal lesions must be thoroughly examined histologically because of potential overlapping features with malignant mesothelioma. The morphologic spectrum of malignant mesothelioma and its usually straightforward distinction from müllerian neoplasms is considered, as is its occasional presentation as a dominant ovarian mass. The spectrum of low-grade serous peritoneal neoplasms including the “psammocarcinoma” is reviewed. Finally, various benign müllerian lesions, particularly endometriosis and endosalpingiosis, may be conspicuous in peritoneal specimens and sometimes are grossly striking. The usual presence of benign endometrioid epithelium and stroma should facilitate the correct diagnosis of endometriosis, but in cases in which the stroma is atrophic or the sole component (stromal endometriosis), diagnostic problems may arise.