The default network is a set of brain regions that become active during rest and internally directed mental processes. It plays a key role in internal mentation, including spontaneous thoughts, memory, social cognition, and emotion. Recent studies using structural and functional imaging, along with behavioral data on mind wandering, suggest that the default network is composed of interacting hubs and subsystems that support adaptive functions. These subsystems include the medial temporal lobe (MTL), which is involved in memory, and the dorsal medial prefrontal cortex (dMPFC), which is involved in introspection about mental states. The anterior medial prefrontal cortex (aMPFC) and posterior cingulate cortex (PCC) are also key hubs that contribute to the valuation of personally significant information. The default network is functionally distinct from the task-positive network, which is involved in external attention. While the default network is often associated with passive states, it also supports goal-directed cognition, particularly in tasks involving memory, social cognition, and emotion. The network's adaptive role in internal mentation is supported by evidence showing that it helps individuals simulate the past and future, reflect on the mental states of others, and process personally significant information. The default network's functions are further supported by its connectivity with other brain regions and its involvement in memory consolidation and emotional processing. Overall, the default network is a complex system that plays a crucial role in various cognitive and emotional processes.The default network is a set of brain regions that become active during rest and internally directed mental processes. It plays a key role in internal mentation, including spontaneous thoughts, memory, social cognition, and emotion. Recent studies using structural and functional imaging, along with behavioral data on mind wandering, suggest that the default network is composed of interacting hubs and subsystems that support adaptive functions. These subsystems include the medial temporal lobe (MTL), which is involved in memory, and the dorsal medial prefrontal cortex (dMPFC), which is involved in introspection about mental states. The anterior medial prefrontal cortex (aMPFC) and posterior cingulate cortex (PCC) are also key hubs that contribute to the valuation of personally significant information. The default network is functionally distinct from the task-positive network, which is involved in external attention. While the default network is often associated with passive states, it also supports goal-directed cognition, particularly in tasks involving memory, social cognition, and emotion. The network's adaptive role in internal mentation is supported by evidence showing that it helps individuals simulate the past and future, reflect on the mental states of others, and process personally significant information. The default network's functions are further supported by its connectivity with other brain regions and its involvement in memory consolidation and emotional processing. Overall, the default network is a complex system that plays a crucial role in various cognitive and emotional processes.