Microtubule-associated protein tau (MAP-tau) plays an important role in stabilizing neuronal structure, in axonal transport, regulating cellular division, and apoptotic processes. In recent years the structure and functions of MAP-tau, as well as its physiological changes and pathology, have been extensively studied. Degenerative diseases of the central nervous system having MAP-tau involved in their pathogenesis are called tau-pathies. This group includes Alzheimer's disease, frontotemporal dementia, progressive supranuclear palsy, and corticobasal degeneration. The authors present the clinical pictures of these diseases (with the dominant features of dementia), and current possibilities in their diagnosis and treatment. MAP-tau as a marker for neurodegenerative process is already used in the diagnosis of tau-pathies. Further studies on its pathology may also give rise to some new therapeutic strategies.