The new WHO series on Pathology and Genetics of Tumours of the Breast and Female Genital Organs (33) defined to name the group of epithelial ovarian tumors with a dignity between benign and malignant exclusively as Borderline Tumors of the Ovary (BOT) and to skip the term "... of low malignant potential". Further, the term "atypical proliferative tumour" was not recommended by the WHO. During a Consensus Meeting on Borderline Tumors of the Ovary held at Bethesda on August 27-28, 2003 (2) the expert panel also recommended to use the BOT terminology. However the term "tumour of low malignant potential" and--less favourable--"atypical proliferative tumour" may be used as synonym. Both groups agreed unanimously that the name carcinoma should not be included in the diagnosis. The group of borderline ovarian tumors is heterogeneous. 80 to 90% of the cases have a very favourable prognosis while 10-20% exhibit a recurrent clinical course with peritoneal implants and very rarely death from the tumor within 10 years. The morphological criteria and supporting methods for recognizing unfavorable BOT and for distinguishing them from highly differentiated ovarian carcinomas are summarized. The prognostic importance of peritoneal implants is described. The concept of micropapillary serous carcinomas (MPSC) and its implications on the diagnostic work of pathologists will be discussed. The presented data focus on serous tumors since this is by far the most common variant.