Abundant cytoplasmic inclusions consisting of aggregated hyperphosphorylated protein tau are a characteristic pathological observation in several neurodegenerative disorders such as Alzheimer's disease, Pick's disease, frontotemporal dementia, cortico-basal degeneration and progressive supranuclear palsy. The recent finding that mutations in the tau gene are responsible for frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) has provided convincing evidence that tau protein plays a key role in neurodegeneration. In the short period since the identification of pathogenic mutations in tau, remarkable progress has been made in understanding some of the mechanisms by which these mutations lead to neurodegeneration. Understanding the disease processes will hopefully provide us with new leads in developing effective therapies for dementia.