Thyroid cancer, once considered relatively uncommon in the general population, is now the eighth most commonly diagnosed cancer among women worldwide, and the third most common cancer among women under 45 years of age. The incidence is substantially higher in women than men (3:1 ratio); this differential is highest between ages 15 and 39 and declines with age. Nearly all thyroid cancers derive from the follicular epithelium, and the most common histological type is papillary thyroid cancer (PTC). Incidence of thyroid cancer has been increasing in many countries since the early 1980s. This trend appears to be attributable to a combination of diagnostic changes, improvements in the detection and diagnosis of smaller PTCs, and changes in the prevalence of environmental factors. While less common, the incidence of larger, more advanced-stage PTCs has increased at a similar rate to that of smaller PTCs.