Hashimoto's Thyroiditis (HT) is the most common cause of thyroid diseases in children and adolescents and it is also the most common cause of acquired hypothyroidism with or without goiter. The linkage between HT and some HLA genes has been reported and a genetic predisposition to thyroid autoimmunity is suggested by observations in twins. There is no direct evidence that infections cause HT in humans, while iodine and iodine containing drugs can precipitate HT in susceptible populations. There is an infiltration of lymphocytes and plasma cells between the follicles followed by their atrophy. The clinical course is variable and spontaneous remission may occur in adolescence. Goiter, menstrual disorders, short stature, constipation, nervousness and exophthalmos have been reported as the most recurrent clinical features of HT. Nevertheless we studied 33 patients with HT, 22 girls and 11 boys aged 4.9-19 years and most of them were euthyroid clinically. Hashimoto thyroiditis is often associated with type 1 diabetes and other autoimmune disorders such as coeliac disease, type 2 and type 3 polyglandular autoimmune disorders. Girls with Turner syndrome may develop HT. Patients with HT have positive antibodies to thyroglobulin and/or to thyroperoxidase in blood. Thyroid function could be normal or abnormal (overt hypothyroidism, subclinical hypothyroidism and hyperthyroidism). Abnormal ultrasound patterns may be present in patients with HT disease as diffuse hypoechogenicity and pseudonodules. L-thyroxine therapy is indicated in HT with hypothyroidism, but periodic re-evaluations are required because HT could be a self-limited disorder in some cases.