Brain aging is characterized by numerous physiological, structural, functional, and neurocognitive changes. The interplay of these various processes is complex and characterized by large interindividual differences. Although much is not understood about how we age, there are numerous studies detailing the nature of the changes in the brain as we age. This chapter will review some of the functional, neuropsychological, neuroimaging, and immunological processes known to occur in normal or "healthy" aging. Large epidemiological studies of older adults have shown health status in the elderly is a function of the negative consequences and impairment in functional performance caused by medical co-morbidities. Immunological function declines with age, such that adaptive immunity is reduced to previously encountered pathogens, and there is a weakened adaptive immune response to novel pathogens. Structural and functional neuroimaging studies of cognitively intact older adults have consistently shown volume loss and loss of white matter structural integrity, particularly in prefrontal cortex, which may be associated with cognitive decline. While data are incomplete, one consistent finding has been a decline in cognitive domains, such as arithmetic/numerical ability and perceptual speed. Alternatively, other cognitive functions such as verbal ability, word knowledge, and semantic memory remain quite preserved even to old age. Factors identified for healthy cognitive (brain) aging are multifactorial, and likely incorporate biological systems as well as cognitive reserve.