OBJECTIVE Thyroid cancer incidence is increasing worldwide. Causes are highly debated. RESULTS Thyroid cancer increase has been associated to socioeconomic status, better access to healthcare and rising use of thyroid imaging. Therefore, the rise could be apparent because of the useless identification of a large reservoir of subclinical papillary lesions that will never affect patient health (overdiagnosis).However, not all epidemiological and clinical data support this hypothesis. The increasing number of large tumors, the increasing thyroid cancer-related mortality in spite of earlier treatment and the changes in thyroid cancer molecular profile suggest a true increase. Recently increased and thyroid-specific environmental carcinogens could be responsible, such as radiation (mostly medical radiation), increased iodine intake and chronic lymphocytic thyroiditis and environmental pollutants such as nitrates, heavy metals and other compounds largely used in the industrialized society. Possible mechanisms await further investigation. CONCLUSIONS The increased incidence of thyroid cancer is the likely result of two coexisting processes: increased detection (apparent increase) and increased number of cases (true increase) due to unrecognized thyroid-specific carcinogens.To identify causal factors and to differentiate stationary cancers from those that will progress are major urgent requirements for both clinical and scientific purposes.