The ovary is a common site of metastatic tumour. In many cases of ovarian metastasis there is a known history of malignancy but in other cases the ovarian tumour is the first manifestation of disease. In this review metastatic colorectal, appendiceal, gastric, breast, pancreatic and biliary tract, hepatocellular, renal, transitional and cervical carcinomas and metastatic malignant melanoma involving the ovary are discussed, as is the issue of synchronous ovarian and endometrial carcinomas. Peritoneal tumours, including primary peritoneal carcinoma, mesothelioma and intra-abdominal desmoplastic small round cell tumour, involving the ovary are also discussed, together with a variety of other rare, metastatic ovarian neoplasms. Many metastatic adenocarcinomas involving the ovary, especially those exhibiting mucinous differentiation, closely mimic primary ovarian adenocarcinomas with morphologically bland areas simulating benign and borderline cystadenoma. This is referred to as a maturation phenomenon. In recent years immunohistochemistry, especially but not exclusively differential cytokeratin (CK7 and CK20) staining, has been widely used as an aid to distinguish between a primary and secondary ovarian adenocarcinoma. While immunohistochemistry undoubtedly has a valuable role to play and is paramount in diagnosis in some cases, the results must be interepreted with caution, especially in mucinous tumours, and within the relevant clinical context. We feel the significance of differential cytokeratin staining is not always understood by histopathologists and this can result in erroneous interpretation. We critically discuss the value of immunohistochemistry and associated pitfalls with each tumour type described.