The characteristic scanning electron microscopic features of the normal thyroid gland, benign thyroid lesions such as nodular (adenomatous) and colloid goitre, adenomas and thyroiditis, and malignant tumors such as papillary carcinoma, follicular carcinoma, anaplastic carcinoma and medullary carcinoma are described. One or more cilia are present in the center of the follicular surface of almost every epithelial cell in the normal thyroid gland as well as in most goitres. Their number is reduced in adenomas and differentiated carcinomas. Medullary carcinomas and anaplastic carcinomas usually lack cilia. Variation in distribution and appearance of microvilli seems to be related to functional differences in the normal thyroid and goitres. In neoplastic conditions the abundance of microvilli steadily decreases from ordinary papillary carcinomas to follicular variants of papillary carcinoma and to follicular carcinoma. Most of the cells in medullary carcinoma and anaplastic carcinoma have few or no microvilli. Benign and neoplastic Hürthle cells have a very characteristic appearance. Distinct, smooth-surfaced cells are interspersed among cells rich in microvilli. The literature is reviewed. Our own experience from examinations of 264 thyroid specimens is included.