This review of human autoimmune thyroid disease (AITD) focuses mainly on the epidemiology and pathophysiology of this very common disorder, although some specific clinical situations are discussed. One peculiarity of AITD is the existence of two contrasting phenotypes: hypothyroid thyroiditis and hyperthyroid Graves' disease. Graves' disease is characterized by the presence of anti-TSH receptor antibodies capable of activating the TSH receptor, leading to thyroid hypertrophy and hyperfunction. In contrast, autoimmune thyroiditis progresses slowly, through necrosis/apoptosis of thyroid cells and their functional impairment. Other forms of autoimmune thyroiditis such as postpartum thyroiditis and silent thyroiditis are also described. The aim of this non exhaustive review is to provide the interested reader with basic information required for further investigation.