This book explores the major aspects of the physiological functions of tau protein, which, despite the historical focus on Alzheimer's disease, has only recently emerged as a promising therapeutic target for Alzheimer's treatment and prevention, as well as for other neurological conditions collectively known as tauopathies.  Also covered is the role of tau protein in microtubule dynamics, neuronal signaling, synaptic function, and regulation by post-translational modifications. The book also discusses tau's presence and roles in non-neuronal cells, as well as the pathological changes in tau that are associated with neurodegenerative diseases. The final section looks at future research directions in this field.