Abundant tau-positive neurofibrillary lesions constitute a defining neuropathological characteristic of Alzheimer's disease. Filamentous tau pathology is also central to a number of other dementing disorders, such as Pick's disease, progressive supranuclear palsy, corticobasal degeneration and familial frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17). The discovery of mutations in the tau gene in FTDP-17 has firmly established the relevance of tau pathology for the neurodegenerative process. Experimental studies have provided a system for the assembly of full-length tau into Alzheimer-like filaments, providing an assay for the testing of compounds that inhabit the formation of tau filaments.