Autoimmune thyroid diseases (AITD) are the far most common autoimmune disorders, their prevalence in Western countries exceeding 5% of the general population. In the large majority of individual cases the clinical impact of AITD is not severe, however, their widespread diffusion renders them a significant health problem. AITD are heterogeneous in their clinical presentation: the two main forms are autoimmune thyroiditis (AT) and Graves’ disease (GD). Although they probably share, at least in part, a common genetic background and may occur in the same family as well as in the same individual, they are definitely two distinct diseases both in their clinical presentation and their pathophysiology. In fact, AT causes structural thyroid damage (mainly via cell-mediated immune destruction of thyroid follicular cells) which results, as a rule, in functional impairment (hypothyroidism); however, depending on clinical variants, evolution towards hypothyroidism may be very low, or thyroid function impairment occurs after an initial phase of mild thyrotoxicosis due to relatively rapid gland destruction. GD patients have hyperthyroidism, often severe, due to autoantibody-mediated thyrotropin receptor stimulation, with thyroid cell hyperplasia and hyperfunction. Such a functional heterogeneity is a key feature for the clinical management: as a matter of fact, therapy of AITD is mainly therapy of thyroid dysfunction. Moreover, since hyperthyroidism is quite early perceived by the patient as a cause of discomfort, the timing of the natural history of GD is relatively well defined; on the other hand, AT may be asymptomatic for a long time and defining its natural history in a single patient may be difficult. In some AITD patients (mainly, but not exclusively, with GD), clinical features not directly related to thyroid dysfunction, such as orbitopathy, are present. Graves’ orbitopathy is probably related, at least in part, to autoantibodies directed to thyrotropin receptor; it may be, in a minority of patients, severe and sight-threatening, and represents an independent clinical problem.