Abnormal protein aggregation is a common characteristic of many neurodegenerative diseases of the brain. Filamentous deposits made of the microtubule-associated protein tau constitute a major defining characteristic of several neurodegenerative diseases known as tauopathies. The role of tau in neurodegeneration has been clarified by the identification of genetic mutations in the tau gene in cases with familial frontotemporal dementia and parkinsonism linked to chromosome 17. Furthermore, some sporadic tauopathies are associated with tau gene polymorphisms. Although it is still debated how tau gene mutations lead to neuronal death, it is clear that different mutations lead to tau pathologies with characteristics similar to those found in sporadic tauopathies. These findings have definitely shown that in tauopathies tau aggregation is directly associated with development of neurodegeneration and neuronal death.