Graves' thyroid disease and the orbitopathy which often accompanies it have puzzled physicians and scientists for more than 150 years. Although certain histopathological features have been documented, it is not known what mechanisms are responsible for the systemic and/or ocular disorders. The ocular condition characteristic of Graves' disease may exist in the absence of clinical or biochemical evidence of thyroid dysfunction, and when the systemic and ocular condition exist together, they may follow completely different clinical courses. Because the relationship between the thyroid disorder and the orbitopathy is not established, the authors refer not to "thyroid ophthalmopathy," but to "Graves' ophthalmopathy." In this review, histopathological features of Graves' ophthalmopathy are reviewed. The differential diagnosis of the ophthalmopathy and approaches to its evaluation are discussed, notably the T-4 radioimmunoassay, T-3 resin uptake and TSH tests, as well as CT scanning and ultrasonography. The characteristics of Classes 0-6 ophthalmopathy are outlined and the management of cases in each class is considered. The possible immunological basis for Graves' ophthalmopathy is also reviewed.