Epithelial ovarian cancer is one of the most lethal gynecological malignant tumors despite improvement of the treatment. Recent molecular studies show that ovarian cancer is a heterogeneous disease which is reflected by different histologic types. These subtypes differ from their origin, pathogenesis and molecular alterations and can be divided in two major groups. The type I cancer (low grade) evolves from precursor lesions in a step-wise process. In contrast, the type II cancer (high grade) grows rapidly without any identifiable precursors. Among all subtypes is heterogeneity in the biological behavior which has implications in patient prognosis and treatment especially for individualized therapies in the future.