Surface epithelial neoplasms of the ovary, which represent about two-thirds of all primary ovarian neoplasms and almost 90% of all malignant ovarian tumors, are a variegated and heterogeneous collection of proliferations. The standard classification of ovarian neoplasms is the system developed by the World Health Organization in 1973; various studies, however, have found that observer variability for problematic cases is substantial. Prognosis and quality assurance make this variability a major concern. The authors examine the reasons behind this variability, the theories behind classification systems in general, problems of taxonomy, and the significance of confidence intervals. Appendices develop more fully the authors' approach to problems of classification and interobserver agreement, with notes on specific differentiated types of ovarian neoplasms.