Converging evidence suggests that when individuals are left to think to themselves, a so-called default network of the brain is engaged, allowing the individual to daydream, reflect on their past, imagine possible future scenarios, and consider the viewpoints of others. These flexible self-relevant mental explorations enable the anticipation and evaluation of events before they occur, and are essential for successful social interactions. Such self-projective efforts are particularly vulnerable to disruption in frontotemporal dementia (FTD), a neurodegenerative disorder involving damage to the frontal and temporal lobes of the brain. In this Review, we explore how the progressive degeneration of the neural networks in two subtypes of FTD—the behavioral variant and semantic dementia—affects key structures of the default network and putative self-projective functions. We examine the available evidence from studies of autobiographical memory, episodic future thinking, theory of mind, moral reasoning, and economic decision-making in these neurodegenerative diseases. Finally, we propose that the mapping of default-network functions onto discrete subsystems of the default network may need revision in light of neuropsychological and clinical evidence from studies in patients with FTD.