Thyroid cancer is the most common endocrine malignancy and the seventh most common cancer among women.The age-adjusted incidence rates for thyroid cancer have increased worldwide for decades.According to a SEER report,thyroid cancer has the fastest reported increase in age-adjusted annual incidence rates compared to other types of malignancies during a recent 14 year period, from 3.4/105 in 1994 to 6.6/105 in 2008 (94%) in men, and from 8.7/105 in 1994 to 19.7/105 in 2008 (126%) in women in the United States.The increased incidence rates have been largely limited to papillary thyroid cancer, which accounts for about 90% of all thyroid cancer cases in the United States.Use of radiation to treat the head and neck for benign childhood conditions in 1920-1950 is thought to have been responsible for the observed rapid increase in incidence that occurred before 1975.Although changes in diagnostic procedures or increased medical attention to thyroid nodules have been suggested to underlie the rise during the past two decades, unknown environmental exposures have also been suggested and may also account for the increasing trend.The etiology of thyroid cancer is largely unknown.The few established risk factors including radiation exposure, family history, and benign thyroid diseases can explain only a small portion of the thyroid cancer cases, which leaves a majority of the cases unexplained.As such, preventive measures for this disease are unknown.The presentation will discuss the time trends of thyroid cancer in the U.S.and other parts of the world, the controversies of increasing trends, current understanding of the etiology of thyroid cancer, and major hypotheses underlying the observed increasing time trends.