The etiopathogenesis and the classification of oncocytic (Hürthle cell) tumors of the thyroid is reviewed with an emphasis on the role played by mitochondrial and nuclear genetic abnormalities that interfere with mitochondrial function. Oxyphilia is classified into primary or secondary and the so-called Hürthle cell carcinoma is divided into oncocytic (Hürthle cell) variants of papillary and follicular carcinoma.