Clear cell cancers of the ovary are rare and constitute about 6% of ovarian tumours, except in Japan, where the incidence is higher. The tumour has distinctive morphological, molecular and clinical features. There is an association with endometriosis. The cancer is distinct from ovarian endometrioid tumours as well as endometrial cancer. Primary management is surgery as for more common ovarian cancers. A key feature is a mutation in the ARID1A gene, which regulates gene activity by altering DNA structure. Clear cell cancers have a poor prognosis when at an advanced stage and respond poorly to conventional cytotoxic chemotherapy. Immunotherapy has to date shown little promise. Despite some unique molecular features, targeted therapies have yet to lead to an effective treatment.