Mucinous ovarian tumors are among the most difficult ovarian neoplasms for surgical pathologists to interpret. Approximately 20% of primary ovarian mucinous tumors are borderline tumors, noninvasive (intraglandular; intraepithelial) carcinomas, or invasive carcinomas; the remainder are cystadenomas. The borderline tumors may be of intestinal type or mullerian (endocervical-like) type. The intestinal-type tumors are by far the most common. Their frequently heterogeneous composition with coexisting elements of cystadenoma, stromal microinvasion, noninvasive carcinoma, and invasive carcinoma requires careful gross examination and extensive sampling of the tumors. The inherent glandular complexity of proliferating mucinous tumors complicates recognition of stromal invasion. Some mucinous carcinomas with expansile (confluent) invasion may be very difficult to discriminate from extensive noninvasive carcinoma. Interobserver reproducibility probably requires use of an arbitrary minimum size criterion for the diagnosis of expansile invasion. Primary invasive carcinomas with an infiltrative growth pattern are less common. Rarely, distinct mural nodules of reactive or neoplastic type are found in the cystic wall of a mucinous tumor. Pseudomyxoma peritonei almost never results from a ruptured primary ovarian mucinous neoplasm, but often produces secondary borderline-like ovarian tumors with prominent pseudomyxoma ovarii. Prognosis of mucinous tumors is highly dependent on stage and histologic composition. Borderline tumors, noninvasive carcinomas, microinvasive tumors, and invasive carcinomas with an expansile growth pattern are generally stage I and have an excellent prognosis with only occasional examples of metastatic spread. Invasive carcinomas with an infiltrative growth pattern are more aggressive, accounting for almost all high-stage mucinous tumors, and are responsible for most deaths caused by tumor. A high index of suspicion that a mucinous tumor is actually a metastasis from another organ is required by pathologists and gynecologists to prevent misdiagnosis of a metastatic neoplasm as a primary ovarian tumor. Secondary mucinous tumors are significantly more often bilateral, <10 cm in maximal dimension, and of high stage. Numerous immunohistochemical stains proposed to aid in the differential diagnosis of primary vs. secondary mucinous tumors also are reviewed.