Abundant neurofibrillary lesions made of abnormal and hyperphosphorylated microtubule-associated protein tau constitute one of the defining neuropathological features of Alzheimer's disease. However, tau containing filamentous deposits in neurons and/or glial cells also define a heterogeneous group of neurodegenerative disorders clinically characterized by dementia and/or motor syndromes. Thus, all these disorders are collectively grouped under the generic term of tauopathies. In the present review we outline the morphological and biochemical characteristics of some major tauopathies, including Alzheimer's disease, Pick's disease, progressive supranuclear palsy, corticobasal degeneration and argyrophilic grain disease. The second part will deal with the recent discovery of tau gene mutations in frontotemporal dementia and parkinsonism linked to chromosome 17 which demonstrates that tau dysfunction can lead to neurodegeneration. Finally, we will discuss the very recent finding of 'tau-deficient' tauopathy in a subset of frontotemporal dementia cases.