Studies in children medically exposed to external irradiation more than 50 years ago revealed a considerably increased risk for thyroid cancer. Similarly, a strongly age-dependent risk for thyroid cancer was observed in the Japanese population after the atomic bomb explosions with the highest risk in the group of children below age of 10. After the Chernobyl accident, children from Belarus living in highly exposed regions received mean thyroid doses by radioactive fallout higher by a factor of approximately 2 as compared to the survivors of the atomic bomb explosions. This lead to a radiation related increase of thyroid cancer incidence in children and adolescents with the highest incidence in age group 0-4 years up to now totally amounting to approximately 5 000 cases. For screening of thyroid cancer in children, high resolution ultrasound is the method of choice which has to be complemented by fine-needle aspiration biopsy in suspicious cases. Diagnostic criteria for malignancy in childhood thyroid cancer by ultrasound are hypoechogenicity and irregularity of the outline, subcapsular location of lesions and increased peri-intranodular vascularisation. The treatment strategy for thyroid cancer in children does not differ substantially from the approach used in adults. Primary treatment consists of thyroidectomy and lymph node dissection. Careful and complete removal of the lymph nodes is of great clinical relevance in children because of very frequent node involvement (between 40% and 90%). Because of the high prevalence of lymph node metastases, ablation of thyroid remnants is mostly indicated in children with thyroid cancer. Distant metastases which need higher activities of radioiodine are less frequent with 10-20%. Even in advanced cases of childhood thyroid cancer, long-lasting remissions can be achieved. A specific finding in children is disseminated, milliary lung metastases with intense radioiodine uptake. In this situation, pulmonary fibrosis may be a severe side-effect so that the indication for repeated courses of radioiodine therapy has to be decided thoroughly. With respect to side-effects of radioiodine therapy, the risk of developing breast cancer has to be taken into account seriously since especially the female breast is exposed to a relatively high radiation dose. Generally, young patients treated with high activities of radioiodine should be carefully followed up during their whole lifespan.