This report focuses on the borderline category of ovarian mucinous tumors and summarizes the points of general agreement and persistent controversies identified by experts in the field who participated in the Borderline Ovarian Tumor Workshop held in Bethesda, MD, in August 2003. Points of agreement and persistent controversies regarding nomenclature, diagnostic criteria, and behavior are addressed for the following ovarian mucinous tumor categories: mucinous borderline ovarian tumor (M-BOT; synonymously referred to as atypical proliferative mucinous tumor of ovary or mucinous ovarian tumor of low malignant potential), M-BOT with intraepithelial carcinoma, and M-BOT with microinvasion. The morphologic spectrum of M-BOTs with regard to distinction from mucinous cystadenoma and the confluent glandular/expansile type of invasive mucinous carcinoma is also addressed. Non-ovarian mucinous tumors, including the secondary ovarian mucinous tumors associated with pseudomyxoma peritonei and metastatic mucinous carcinomas with a deceptive pattern of invasion, are recognized as tumors that can simulate primary M-BOTs. Improved classification of these mucinous tumors has clarifed the behavior of true M-BOTs by excluding these simulators from the M-BOT category.