Aging is a complex biological process characterized by an accumulation of molecular and cellular damage over the lifespan. The body’s inability to repair this damage leads to a subsequent loss of physiological functions. These include sensory, motor, and cognitive functions that, when impaired, impact the quality of life. Age is also a major risk factor for many life-threatening diseases including cancer, cardiovascular disease, and neurodegenerative disorders. Brain age is a biomarker that predicts chronological age using neuroimaging features. The brain is susceptible to the effects of aging, manifesting as changes in structure and cognitive function. Neuroimaging technologies, including magnetic resonance imaging (MRI), have made it possible to monitor these changes in vivo. The most common changes associated with aging are brain atrophy (i.e., loss of grey matter volume and cortical thinning), a reduction in white matter integrity and volume, and abnormal functional connectivity. When severe, these phenotypes can be considered a sign of accelerated aging or an underlying disease process. The difference between predicted biological and actual chronological age signifies a deviation from the typical aging trajectory and has the potential to identify individuals with disease, monitor treatment effects, or identify lifestyle factors that are beneficial or detrimental to brain health.