The study of rare, inherited forms of different diseases resulted in the discovery of gene defects that cause inherited variants of the respective diseases. The defective genes were found to encode major molecular players leading to the neuropathological lesions or factors that characterize these diseases. The exact role of the tau protein in the neurodegenerative process is still under debate. It is very important to understand the normal biological roles of tau and the specific events that induce tau to become neurotoxic. Tau is the major microtubuleassociated protein (MAP) of a mature neuron. The other neuronal MAPs are MAP1 and MAP2. These three MAPs perform similar function, promoting assembly and stability of microtubules. Tau protein was isolated as a microtubule-associated factor in the porcine brain. It was isolated as a protein that co-purified with tubulin and had the ability to promote microtubule assembly in vitro. Normal adult human brain tau contains 2–3 moles phosphate/mole of tau protein. Hyperphosphorylation of tau depress this biological activity of tau. Almost 80 diseases caused by missense mutations and intronic mutations in the tau gene have been found in familial cases of frontotemporal dementia (FTD). In Alzheimer’s disease (AD), there are intraneuronal neurofibrillary tangles composed of the microtubule-associated protein tau (MAPT). In other neurodegenerative diseases, there are similar deposits of tau, in the absence of extracellular deposits (progressive supranuclear palsy, corticobasal degeneration, argyrophilic grain disease, etc.). Tau pathology is also often seen in some forms of Parkinson’s disease (PD) and prion diseases. In genetic forms of FTD, mutations in tau implicate abnormal tau as the initiation of neurodegeneration. In FTD, there are deposits especially in temporal and frontal lobes, regions that are very important for behavior and executive function. It is critical to understand how tau becomes pathogenic, in order to consider developing any strategies for treatment.