Autoimmune thyroiditis (AIT) is the most common thyroid disorder in the pediatric age range. The disease results from an as yet poorly characterized defect or defects in immunoregulation and a cascade of events progressing from lymphocyte infiltration of the thyroid, to T-cell- and cytokine-mediated thyroid follicular cell injury, and apoptotic cell death. Approximately 70% of disease risk has been attributed to genetic background with environmental factors being important in triggering disease in susceptible individuals. The contribution of individual genes is small and probably polymorphisms in multiple genes play a role. Some immuno susceptibility genes affect immune recognition or response in general, while others are thyroid-specific. Environmental agents may act through an epigenetic mechanism. Antibodies (Abs) to a variety of thyroid-specific antigens are detectable in a majority of patients, but the role of Abs in mediating cell injury and death is unclear and only thyrotropin (TSH) receptor Abs significantly affect thyroid function by interfering with (or stimulating) the action of TSH. Nonetheless, thyroid peroxidase (TPO) Abs and thyroglobulin (Tg) Abs, present in a majority of patients, are valuable diagnostically as markers of underlying autoimmune thyroid destruction. TSH receptor blocking Abs are found in ~18% of children and adolescents with severe hypothyroidism and, when persistent, may identify an adolescent likely to have a baby with TSH receptor blocking Ab-induced congenital hypothyroidism. AIT may coexist with other organ-specific autoimmune diseases. Although the most common age at presentation is adolescence, the disease may occur rarely in children <1 year of life.