Alzheimer's disease (AD) is the most common type of neurodegenerative disorder in the ageing population, with dementia as a common consequence. AD is defined pathologically by the appearance of extracellular senile plaques and intracellular neurofibrillary tangles, as described by Alois Alzheimer about a century ago. The causes for AD include genetic predisposition in a small population, ageing and environmental stresses in majority cases. The underlying pathogenic cascades, increases in expression of amyloid precursor protein and accumulation of Aß and reactive oxidant activity and inflammation, have the features of both adaptive, at least initially, and harmful when becoming excessive. Dementia, on the other hand, is a clinical diagnosis and is defined as globally, persistently impaired cognitive skills including memory. Alzheimer dementia refers to clinical dementia in patients who also have Alzheimer neuropathology. Alzheimer dementia is what brings the patients to seek medical treatments. An extraordinary inability to form new memory, especially of those episodic type, and executive dysfunction are among the earliest symptoms in AD patients. In end-stage AD, cognitive degeneration extends far beyond memory loss. The underlying causes include decreases in impaired brain metabolism, which results in impaired synaptic functions and capacities, thus impaired information processing, and eventually leads to neuronal injury and death. This book presents leading-edge research in this dynamic field.