The nervous system is a highly complex network of nerves that coordinates actions and sensory information by transmitting signals to and from various parts of the body. Random molecular damage that steadily accumulates with age drives the ageing process at the cellular level. Many molecular mechanisms are involved and exogenous factors, like stress, also play a role in the aging process. The nervous system at the cellular level comprises a special cell called a neuron, which transmits signals to the other cells. Many physical, chemical, or biological changes in the status of neurons characterize brain aging. It is manifested in the form of deterioration in cognitive function and dementia. Dendritic regression in pyramidal neurons, synaptic atrophy, decrease of striatal dopamine receptors, accumulation of fluorescent pigments, cytoskeletal abnormalities, and reactive astrocytes and microglia are the common features of brain aging. The consequence of molecular and cellular alterations in brain aging is characterized by a plethora of anatomical changes, such as significant neuronal loss in hippocampus and neocortex. This book provides comprehensive insights on the molecular and cellular mechanisms of aging. The readers would gain knowledge that would broaden their perspective about this area of study.