Oncocytic (Hurthle cell) lesion is defined as a hypercellular lesion with a predominance of Hurthle cells (more than 75%) and a paucity of colloid on cytological examination. Hürthle cells (oncocytic cells) are large thyroglobulin-producing epithelial cells that are present in both nonneoplastic and neoplastic thyroid lesions, which encompass pathological entities ranging from hyperplastic or adenomatous (nonneoplastic) nodules, including nodular goiter and chronic lymphocytic thyroiditis (Hashimoto’s thyroiditis), to encapsulated neoplasms such as benign follicular adenomas, to malignant neoplastic disease including well-differentiated thyroid cancers, medullary thyroid cancer, and metastases from various sources. Nodules may be discovered on routine physical examination when they begin to cause symptoms of compression, or as incidental findings on imaging studies performed for other reasons. This chapter discusses the thyroid disorders in which oncocytic (Hurthle) cells are encountered during a fine-needle aspiration. These include chronic lymphocytic (Hashimoto’s) thyroiditis, Graves’ disease, nodular hyperplasia of the thyroid gland, and oncocytic thyroid neoplasms. In addition to oncocytic/Hurthle cell adenoma and carcinoma, oncocytic neoplasms include variants of papillary thyroid carcinoma (oncocytic, Warthin-like, tall-cell, and oncocytic follicular), oncocytic poorly differentiated thyroid carcinoma, oncocytic variant of medullary thyroid carcinoma, and oncocytic tumors of non-thyroid origin (granular cell tumor, metastatic and invasive oncocytic tumor, as well as oncocytic parathyroid adenoma).