The most common cell types in clear cell carcinomas (CCC) of the ovary are clear cells and hobnail cells. Clear cells are arranged in solid nests, clusters, or sheets. They are polygonal in shape, have distinct cytoplasmic borders, and contain clear cytoplasm (due to glycogen; PAS-positive – diastase sensitive). There is moderate to severe cytologic atypia consisting of eccentric hyperchromatic nuclei and prominent nucleoli. Mitotic activity is variably increased. Hobnail cells contain bulbous hyperchromatic nuclei which protrude into the glandular lumens (as shown here). Less commonly seen cell types include flattened cells (with atrophic, benign appearance), oxyphilic cells with abundant eosinophilic cytoplasm, and signet ring-cells with intracytoplasmic mucin. All CCCs of ovary are currently considered high-grade by default.