What happens in the brain when the mind wanders? The surprising discovery of the default mode network came hand-in-hand with a trend toward analyzing intrinsic brain activity. Several imaging modalities have been used to isolate and analyze this network, its normal metabolism, development, and functional anatomy. Activations of the default mode network are associated with self-referential thinking, theories of mind, moral reasoning, and mental time travel, while perturbations of it are associated with psychiatric conditions such as PTSD, depression, autism, and Alzheimer's disease. A new language is emerging which describes both normal and pathological brain functions in terms of distributed networks of connectivity rather than discrete and specialized regions.