Secondary (metastatic) neoplasms to the ovary often cause diagnostic problems, especially those tumors that produce large, symptomatic ovarian tumors that masquerade clinically and pathologically as primary ovarian tumors of surface epithelial type. Most of these tumors arise from organs of the digestive system. Except for typical Krukenberg tumors, which usually originate in the stomach and generally are easily recognized, the most diagnostically problematic secondary ovarian tumors are those that originate in the large intestine, appendix, and pancreas. Metastases from these sites typically produce histologic patterns resembling primary ovarian endometrioid carcinoma or mucinous epithelial neoplasms of borderline and malignant types. This review focuses on the diagnostic challenge of distinguishing these secondary ovarian tumors from primary ovarian neoplasms. Studies on useful or potentially applicable immunohistochemical stains are also detailed.