Serous neoplasms of the ovary, which constitute the largest subgroup of the surface epithelial tumors, cluster into three distinctly clinicopathologic groups: benign neoplasms, which are architecturally noncomplex, confined to the ovary, and composed of cytologically bland cells; carcinomatous neoplasms, which have spread beyond the ovary and are cytologically malignant; and an intermediate group, which raises serious problems in taxonomy, differential diagnosis, and prognosis. This chapter focuses on differential diagnosis, emphasizing the authors' experience as well as reports from other investigators.