Tau belongs to the family of microtubule-associated proteins predominantly expressed in neurons where they play an important role in promoting microtubule assembly and stabilizing microtubules. In addition, tau proteins interact with other cytoskeletal elements to allow spacing between microtubules. Recent studies have shown that tau is also actively involved in regulating cell viability and activity. Translated from a single gene located on chromosome 17q21, six isoforms of tau are produced by alternative splicing in adult human brain. Due to multiple post-translational modifications, heterogeneous tau species with a wide range of apparent molecular masses have been observed by denaturing polyacrylamide-gel electrophoresis. Since tau gene mutations and abnormal post-translational modifications have been detected in over 20 neurodegenerative disorders, namely the tauopathies, tau has gained widespread attention as a target protein in Alzheimer's disease and other neurodegenerative disorders. In the present chapter, research progress regarding physiology and pathology of tau is reviewed, particularly in terms of the role of post-translational modification.