The peritoneum is a site for diverse nonneoplastic and neoplastic disease processes, including those encountered in many other organ systems in the body. A recent theory is that Müllerian peritoneal/omental serous carcinoma is a metastasis from a similar tumor of the fallopian tubes, challenging the once-proposed theory that omentum is a part of the secondary Müllerian system and has a role in the pathogenesis of primary serous carcinomas. The metastatic theory applies to serous carcinomas only, not tumors of any other Müllerian cell types. Just being a mere receptacle for metastasis from other organ sites, the notion that the peritoneum and its derivatives are an essential source of primary peritoneal carcinomas remains a valid concept.