Despite its structural simplicity the thyroid gland can give rise to a wide variety of tumor types. Although a substantial amount of work has been done in the past twenty-five years to redefine the clinic-pathological features of most types of thyroid tumors of follicular cell origin, some aspects of thyroid tumors composed of oncocytes, generally known as Hürthle cell tumors, are still matter of discussion. The major issues concern their very existence as a bona fide tumor type, the occurrence of benign and malignant forms and their relative frequency, the predictive value of morphologic parameters, the natural history of the malignant forms and the classification of Hürthle cell tumors with a papillary growth pattern. The definition of oncocytic cell, oncocytic neoplasm and oncocytic carcinoma as well as the pathologic characteristics and clinical behavior of neoplasms composed of oncocytes will be discussed.