The distinction between a primary and metastatic mucinous carcinoma within the ovary may be problematic. In most cases, the distinction can be made by careful pathological examination encompassing both the gross and microscopic findings and taking into account the distribution of the disease. However, immunohistochemistry may be of value in certain scenarios. In this review, I discuss the value of markers in the distinction between primary ovarian mucinous neoplasms and metastatic mucinous carcinomas from the colorectum, appendix, pancreas, biliary tract, stomach and cervix, the most common primary sites which give rise to metastatic mucinous carcinoma within the ovary. There is a significant degree of immunophenotypic overlap between primary ovarian mucinous neoplasms and metastatic mucinous carcinomas from the gastrointestinal tract, especially the upper gastrointestinal type; this is because most primary ovarian mucinous carcinomas and borderline tumours are of so-called intestinal or enteric type and exhibit some degree of positivity with enteric markers. Mullerian type primary ovarian mucinous neoplasms also exist and exhibit distinct immunohistochemical differences to the more common intestinal type.