Insomnia, a psychiatric condition for which diagnosis is typically based on subjective complaints only, has also been associated with several neurobiological factors and mechanisms. In this chapter, the definition of insomnia and its comorbidities is first introduced. The onset of insomnia is illustrated by explaining the mechanisms of natural sleep and how, in insomnia, a combination of psychological and physiological factors can disturb this natural mechanism. Then factors playing a role in particularly the onset, but also the maintenance of insomnia, are explained: hyperarousal, genetic factors and differences in brain structure. The neurobiology of insomnia is then further illustrated by discussing those factors that play a role throughout the day and night: before sleep onset, during sleep, at awakening and during wake. Despite the fact that conclusions are still based on sparse data, the picture of a sleep disorder characterised by a constant state of vigilance that enhances sleep-disturbing psychological factors emerges.