Tauopathy is defined as abnormal accumulation of aberrantly phosphorylated microtubule-associated protein tau in the central nervous system, best demonstrated by immunocytochemistry using anti-tau antibodies. The newly recognized familial tauopathy with mutation in tau gene, that is located on chromosome 17, confirms that the process directly leads to neuronal degeneration. Tau consists of six isoforms translated from alternative splicing of a single gene. They are classified into three repeat (3R) and four repeat (4R) subtypes, by the number of microtubulus-binding domain from the reading or skipping of the exon 10. In sporadic tauopathy, 3R + 4R accumulate in Alzehimer’s disease (AD), 3R in Pick’s disease, and 4R in progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD). In familial tauopathy, the mutations affecting the splicing of the exon 10 accumulate 4R and phenotypically mimick CBD/PSP, while majority of others simulate neurofibrillary tangle-predominant form of dementia (NFTD). Argyrophlic grains (AG) are tau-immunoreactive comma-shaped or filiform structure, and argyrophilic grain dementia (AGD) is a form of senile dementia carrying AG as only morphological substrate explaining dementia. In our consecutive autopsy cases from the oldest old, AGD is the second leading cause of degenerative type of dementia, highlighting the importance of tauopathy in the aging and dementia.