Mutations on tau gene are directly causing a disease named "fronto-temporal dementia with parkinsonism linked to chromosome 17" [FTDP-17]. These mutations [more than 30] have demonstrated the important role of tau pathology in neurodegenerative disorders. Indeed, more than 20 neurodegenerative disorders have a tau pathology, generally with an accumulation of tau proteins in neurons or glial cells, or both. The fact that tau can be directly responsible of diseases and that most dementing disorders have a tau pathology has generated this concept. Tauopathies comprise primary and secondary tauopathies. Primary tauopathies are the diseases with tau playing a major role, such as FTDP-17, PSP, CBD and most fronto-temporal dementia. Secondary tauopathies are diseases like Alzheimer's disease, which is likely a true tauopathy, but fuelled by defects of the APP metabolic pathway.