Functional brain imaging has revolutionized the study of neurological and psychiatric disease. Led by innovations in technology and new analysis techniques, imaging methods are now capable of observing subtle structural changes, functional disruption, and molecular events that precede disease. Here I will focus on the study of aging and dementia to illustrate the potential of melding imaging technology with clinical exploration. Specifically, contrasting normal aging and aging associated with Alzheimer¹s disease has revealed three lessons about the process of brain aging. First, normal aging and Alzheimer¹s disease affects distinct brain cascades that can be formally dissociated from one another. Second, certain cascades are revealed as disruption in the integrity of network interactions. Network disruption may be missed by analyses that focus on certain components of the networks in isolation. Third, Alzheimer¹s disease affects a specific brain network -- called the default network -- that is linked to memory and is spontaneously active much of the time in healthy, young individuals. The observation that the network most active in healthy individuals is the first to show pathology in Alzheimer's disease suggests that activity and metabolic patterns throughout life may set the stage for eventual disease.