Benign and malignant neoplasms of the thyroid are uncommon during childhood, but they create diagnostic problems for the clinician to identify malignant legions that must be removed as well as medically important lesions that require treatment. Throughout the 20th century there was a rapid increase in the incidence of thyroid neoplasms which we now know were induced by ionizing radiation acquired from radiation therapy for benign medical conditions or from environmental sources such as nuclear testing and accidents [Cancer 1961;14:734-743]. Over recent decades, there have been major advances in our understanding of the molecular biology and clinical management of thyroid neoplasms. We hope that the reader of this chapter will find this information of benefit in the clinical management of children with thyroid neoplasms and will be encouraged to study remaining controversial issues. We have divided this chapter into two major sections, the first of which pertains to thyroid nodules and the second to well-differentiated thyroid cancers including papillary, follicular and other variants.