Since their original description in 1929, our knowledge of the natural history and molecular pathology of borderline ovarian tumors (BOTs) has advanced most dramatically over the last decade. It has been estimated that at the time of abdominal exploration for a serous ovarian neoplasm, a BOT will be discovered in approximately 15% of cases. These tumors commonly affect women of reproductive age, have an excellent overall prognosis and the majority are cured with surgery. Nevertheless, a subset of patients with these tumors have an adverse clinical outcome and the molecular features of the associated tumor subtype is currently being defined. These data have caused some to place this more aggressive subtype of BOT, called micropapillary serous with invasive extraovarian implants, into the pathologic continuum with frankly invasive carcinomas. This chapter will explore advances in our understanding of the natural history, clinical and pathologic features of BOTs and will conclude with a discussion of the molecular prognostic factors that might be targets for future therapy. A special emphasis will be placed on points of agreement identified during a recent workshop on BOTs sponsored by the National Cancer Institute (NCI).