When resting or performing cognitive tasks that are either monotonous or short demanding, human beings tend to experience a spontaneous and momentary flow of self-directed thoughts. This particular state of mind, called "wandering mind", commits around 50% of the waking hours in the daily life and reflects the nature of the spontaneous thought, which is current task-independent, and conscious awareness-independent. The content of the inner thoughts is basically characterized by mental images that are self-related and addressed to both the events of the recent past and the prospects of the immediate future, in relation to the preoccupations of the thinking subject. The neuroscientific exploration with brain-imaging, using positron emission tomography and functional magnetic resonance imaging, showed that the wandering mind engages a particular set of brain regions called "default-mode network". This network typically shows increased activity in the resting state of an individual compared to when the same individual performs a cognitive task that requires attention, suggesting its involvement in generating spontaneous internal thoughts. The pervasive and common nature of mind wandering suggests that it serves some important functions in the context of human cognition.