It is about 100 years since the publication of the original description of the disorder now termed as Hashimoto's thyroiditis (HT) or Hashimoto's disease. Hashimoto's thyroiditis has been commonly considered as a well-defined clinicopathological entity, characterized by the presence of goiter and serum thyroid autoantibodies. Very recently, new studies have reported that there is a unique subtype of HT, termed as immunoglobulin (Ig)G4 thyroiditis, which is characterized by lymphoplasmacytic infiltration, fibrosis, increased IgG4-positive plasma cell in the thyroid and high IgG4 level in the serum. Furthermore, from a clinical aspect, IgG4 thyroiditis presents significantly different characteristics from non-IgG4 thyroiditis, with IgG4 thyroiditis being more closely associated with rapid progress, subclinical hypothyroidism, higher levels of circulating antibodies and more diffuse low echogenicity. This review will introduce the most recent concepts about etiologic mechanisms, clinical, imaging and laboratory features of HT and its histological characteristics. Especially, recent advances in recognizing HT with special reference of IgG4-related sclerosing disease (IgG4-RSD) will be summarized. Furthermore, the differences and close relationship between HT and Riedel's thyroiditis (RT), which was also demonstrated to be involved in IgG4-RSD spectrum, will be reviewed.