Thyroid cancer is one of the rarest forms of cancer, and yet there are wide variations in the degree of malignancy, ranging from the most rapidly fatal to the relatively benign. This difference depends almost entirely on the histological type. A "pool" of individuals with occult thyroid carcinomas (in the vast majority of the papillary type) is probably present in most populations even at a young age. Large differences in the estimated frequency of cancer at this site can therefore be caused by variation in diagnostic intensity. Data on changing trends of incidence and mortality are thus subject to reservation, depending on the degree to which they have been influenced by changing diagnostic criteria and the precision of histopathological description. Nevertheless, there is evidence that mortality is slowly falling, whereas incidence is increasing, in several countries. This chapter considers the upward temporal trends of incidence and substantially stable mortality rates for thyroid carcinoma in the past three decades and attempts to interpret these trends in the light of concurrent changes in diagnostic standards and histological classification. Attention will also be drawn to the public health implications of the recent intensive detection and treatment of occult thyroid carcinomas.