Otosclerosis is an important condition for the otorhinolaryngologist, and often in (differential) diagnosis and therapy challenging disease. Although a causal therapy option does not exist until today, if the diagnosis is correct, an operative or apparative hearing rehabilitation with very high chances of success can be promised. In addition to hearing aids surgical treatment with passive implants as a standard therapy, and in special cases also the implantation of active middle ear implants or cochlear implants, in advanced otosclerosis with significant involvement of the inner ear, are of importance. Knowledge of the definition, pathogenesis, diagnostics and in particular of the differential diagnosis as well as the current conservative and operative therapeutic limits are indispensable for the indication and selection of the optimal treatment path.In a two-part presentation, the current specialist medical knowledge of the disease in diagnosis and therapy is summarized. In addition to modern diagnostic procedures, aspects of therapy and prognosis are considered in a second article. This article therefore primarily refers to continuing education for residents in training and for young specialists.