This comprehensive reference book is meant to support clinicians in the diagnosis and treatment of polyendocrine diseases and endocrine neoplastic syndromes. Although a large majority of endocrine diseases present as sporadic cases, an increasing proportion can be identified as part of a polyendocrine or systemic syndrome. These include autoimmune endocrine diseases, which may be part of autoimmune polyendocrine disorders (APS) or rare complex disorders such as POEMS (polyneuropathy, organomegaly, endocrinopathy, M-protein and skin changes) or IPEX (immune dysregulation, polyendocrinopathy, enteropathy, X-linked) syndromes. On the other hand, endocrine tumors may develop in a variety of clinical conditions, including multiple endocrine neoplasia (MEN) syndromes, syndromic diseases such as McCune Albright or Carney's complex, or peculiar familial associations such as pheochromocytoma/paraganglioma syndromes. The book discusses the significant advances that have been made in the clinical and genetic characterization of such entities, with major implications in terms of diagnosis and clinical management - with special attention to emerging syndromes, familial screening , multidisciplinarity and multimodal treatment. This volume is intended for clinicians, residents, specialists and physicians involved in the diagnosis and treatment of affected patients, including specialists in endocrinology, internal medicine, oncology , genetics and imaging.