About 15-20% of all ovarian epithelial neoplasms are of borderline type (or atypical proliferative or carcinoma of low malignant potential) and about 5-7% are mucinous type, which are the second most common type behind the serous borderline tumors. The borderline tumor is a serious diagnostic and treatment problem both for the pathologists and for clinicians. These tumors appeared to be intermediate in their histologic and prognostic features between the benign cystadenomas and clearly malignant carcinomas. The borderline tumors occur most commonly in childbearing age, and show an indolent course. Their prognosis is good, but they are resistant to the traditional chemotherapies. To diagnose the intraepithelial carcinoma, to detect the microinvasion and the expansive invasion in a mucinous borderline tumor, to differentiate from the metastasis of colorectal tumors may be very problematic in the majority of the cases. Eleven cases diagnosed as mucinous borderline ovarian tumor in our institute from 2000 to 2008 were reviewed. Eight out of 11 were intestinal type while three were cervical (mullerian) type. In 5 cases our diagnosis was intraepithelial carcinoma and in 5 cases we found microinvasion. We discuss all of these problems according to the latest literature and our experience, mentioning the problems of the peritoneal and ovarian pseudomyxomas.