Microtubule-associated protein tau is the most commonly misfolded protein in human neurodegenerative diseases, where it becomes hyperphosphorylated and filamentous. Mutations in MAPT, the tau gene, cause approximately 5% of cases of frontotemporal dementia. They are frequently accompanied by parkinsonism. The existence of MAPT mutations has established that dysfunction of tau protein is sufficient to cause neurodegeneration and dementia. However, most tauopathies are not inherited in a dominant manner. The hyperphosphorylated sites are similar between diseases, but filament morphologies and tau isoform compositions vary. This is consistent with the existence of multiple tau conformers and recent findings have provided experimental support for this concept.