The article discusses the role of tau protein in Alzheimer's disease (AD) and related tauopathies. Tau is a microtubule-associated protein (MAP) that promotes microtubule assembly and stabilization. In AD, tau becomes abnormally hyperphosphorylated, leading to its aggregation into paired helical filaments (PHF) and straight filaments (SF), forming neurofibrillary tangles. This hyperphosphorylation is regulated by the degree of phosphorylation, with normal brain tau containing 2-3 moles of phosphate per mole of protein. In AD, tau is ~three to four times more hyperphosphorylated than in normal brains. Abnormally hyperphosphorylated tau sequesters normal tau and MAPs, disrupts microtubules, and self-assembles into PHF/SF. Truncation of tau also promotes its self-assembly. Tau mutations in frontotemporal dementia (FTD) promote abnormal hyperphosphorylation. Inhibition of abnormal hyperphosphorylation of tau is a promising therapeutic target for AD and related tauopathies.The article discusses the role of tau protein in Alzheimer's disease (AD) and related tauopathies. Tau is a microtubule-associated protein (MAP) that promotes microtubule assembly and stabilization. In AD, tau becomes abnormally hyperphosphorylated, leading to its aggregation into paired helical filaments (PHF) and straight filaments (SF), forming neurofibrillary tangles. This hyperphosphorylation is regulated by the degree of phosphorylation, with normal brain tau containing 2-3 moles of phosphate per mole of protein. In AD, tau is ~three to four times more hyperphosphorylated than in normal brains. Abnormally hyperphosphorylated tau sequesters normal tau and MAPs, disrupts microtubules, and self-assembles into PHF/SF. Truncation of tau also promotes its self-assembly. Tau mutations in frontotemporal dementia (FTD) promote abnormal hyperphosphorylation. Inhibition of abnormal hyperphosphorylation of tau is a promising therapeutic target for AD and related tauopathies.