Thyroiditis comprises a group of diseases that can present with strikingly similar symptoms. This frequently poses a diagnostic dilemma to uncover the exact etiology. These patients may present with hyper- or hypothyroid symptoms with subtle differences depending on the etiology. A thorough history-taking and physical examination is the key to uncover these subtle differences. Thyroiditis represents about 20% of all thyroid diseases and is caused by several factors, most commonly autoimmune diseases. Thyroid autoantibodies are mainly directed against thyroid peroxidase (TPO) or thyroglobulin (Tg) and have an association with defined HLA haplotypes implying a genetic predisposition. Iodine therapy, viral infections, pregnancy, menopause, stress, and immune-modulating drugs such as interferon-α have also been linked to autoimmune thyroiditis. Except for Graves’ disease, most cases of autoimmune thyroiditis present initially with hyperthyroidism that returns to euthyroidism or falls to permanent hypothyroidism (subclinical or overt). Clinically, thyroiditis is divided into acute, subacute, and chronic forms, and patients may present either with severe thyroid pain (e.g., acute suppurative thyroiditis, subacute de Quervain’s thyroiditis, radiation-induced thyroiditis, or traumatic thyroiditis) or without evident inflammation, but with goiter or thyroid dysfunction (e.g., silent thyroiditis, Hashimoto’s or Riedel’s thyroiditis). This chapter discusses the epidemiology, etiology, pathophysiology, classification, investigations (work-up), and prognosis (outcome and complications including myxedema coma, thyroid storm, thyroid abscess, septicemia, thyroid cancer, and other complications related to hypothyroidism caused by thyroiditis).