Since its first description in the early part of the twentieth century, struma ovarii has elicited considerable interest because of its many unique features; however, at present a number of aspects remain enigmatic. Although the typical presentation is that of a pelvic mass, unusual clinical manifestations such as hyperthyroidism, ascites, and Meigs' syndrome have been recognised. Uncommon macroscopic and especially histological patterns in struma can cause difficulties in diagnosis. Cystic strumas are challenging to diagnose both macroscopically and histologically. Proliferative changes within struma can be misdiagnosed as cancer. In regard to the occurrence of thyroid-type carcinoma in struma ovarii, precise terminology should be utilised, and the term ‘malignant struma ovarii’ should be avoided because it has been used for several different pathological entities. Papillary carcinoma is the most commonly occurring thyroid-type carcinoma in ovarian struma; however, cases of follicular carcinoma are not infrequent. Histological malignancy in struma does not necessarily equate with biological malignancy, and the majority of thyroid-type carcinomas do not spread beyond the ovary. Strumal carcinoid, a neoplasm apparently unique to the ovary containing elements of both struma and carcinoid, has been misdiagnosed as ‘malignant struma ovarii’ in the past. The differential diagnosis of extra-ovarian spread of struma includes the usual types of thyroid cancer, minimal deviation follicular carcinoma, and peritoneal strumosis. This review emphasises articles both recent and past that have significantly advanced our knowledge of struma ovarii and related neoplasms.