Among well differentiated thyroid tumors, oncocytic neo plasms feature a distinctive set of clinical, morphologic and biologic characteristics, some of which have been a matter of controversy. The world literature on this subject has been reviewed to show that: 1) Morphology accurately predicts the behavior of Hurthie cell tumors assuming that the specimen has been adequately sampled; 2) Capsular and/or vascular invasion is the sine qua non condition for a diagnosis of malignancy; 3) “Indeterminate” or “pos sibly malignant” categories are useless because in the absence of invasion these neoplasms almost invariably behave in a benign fashion; 4) Among cases histologically classified as malignant, a clinically aggressive behavior is to be expected in a high percentage of cases; 5) Size alone cannot be used as a criterion of malignancy; 6) Less than total thyroidectomy provides an adequate treatment for histologically benign tumors; aggressive surgical proce dures (i.e. total thyroidectomy) do not diminish the inci dence of metastasis; 7) Analysis of DNA content may be helpful in defining subsets of patients with Hurthle cell carcinomas having a particulaly poor prognosis; 8) Mito‐chondrial and/or nuclear DNA abnormalities probably play an important role in the cellular alterations which charac terize the phenotype of oncocytes. Acta Pathol Jpn 42: 305–315, 1992.