This chapter presents a state of the art summary of McCune–Albright syndrome (MAS) relation to its biology, epidemiology, disease mechanisms, clinical signs, diagnosis, treatment and prognosis. McCune–Albright syndrome (MAS) is a rare disease resulting from an early embryonic somatic mosaic activating mutation of the GNAS gene. Fibrous dysplasia of bones is the most frequent finding in MAS, which often emerges in the first few years of life and expands during childhood, with no clinically significant bone lesions appearing after age 15 years. Clinical symptoms of MAS involving endocrine organs range from precocious puberty, testicular abnormalities, thyroid disease, FGF23-mediated phosphate wasting, growth hormone excess, to hypercortisolism. Patients with MAS should avoid contact sports and other high-risk activities with significant skeletal involvement, prophylactic optic nerve decompression, surgical removal of ovarian cysts, and radiotherapy for fibrous dysplasia. The prognosis for individuals with MAS varies with disease location and severity.