Understanding of the magnificence and uniqueness of the brain is important for realizing the goals of aging. We must respect its central role in our lives and work to see what we must do to enhance its health throughout life. The brain is precisely responsive to activities and its structure is changed by learning. It is also uniquely sensitive to damage and has a limited capacity for repair, compared to other body parts. What we do changes the brain, and the nature of our mental activities modifies the ability of the brain to resist declines with aging and disease. Cognitive involvement in educational, occupational, and recreational tasks throughout life, as well as physical activities, helps to enhance resistance to decline with age and diseases such as Alzheimer’s. This chapter examines how the four reserves (cognitive, physical, psychological, and social) help to maintain brain function with age. Recent research has uncovered the influence of our partner microbes in the gut on learning, memory, and age-related diseases of the brain. These developments beautifully illustrate how there are things we can all do to maintain brain function with age.