Hashimoto’s thyroiditis is a common autoimmune disorder, which causes significant morbidity. Its pathophysiological hallmark is lymphocytic infiltration of thyroid follicles resulting in autoimmune glandular destruction. Various studies have successfully outlined the genetic and environmental factors responsible for the causation of the disease. In this chapter we will discuss our current understanding of these factors and delineate how Hashimoto’s thyroiditis serves as a paradigm not just for disease of the thyroid gland, but also for autoimmune disease in the human body. Our focus is on the varying presentations of the disease and the relationship between Hashimoto’s thyroiditis and other autoimmune diseases frequently associated with it. The etiological factors and the pathophysiological changes which lead to the development of disease are discussed. Common diagnostic modalities are described, and the need for correlation between the various available diagnostic tests is explained. Various treatment strategies and the appropriate choice for different forms of presentation are discussed. Hashimoto’s encephalopathy, a rare complication, will be addressed separately as its unusual presentation often results in misdiagnosis of the underlying pathology.