Sensory percepts may sometimes occur in the absence of adequate sensory stimulation. The most prominent examples of nonveridical percepts are hallucinations. They can be pathological, but can also affect healthy individuals. Nearly 10% of the general population reported having experienced an unexplained perception; about 3% reported having heard a voice. Hallucinations may result in secondary delusions—inaccurate explanations of what is happening. This entry discusses hallucination in healthy people, in mental disorders, and in other medical conditions; other altered perceptions; nonveridical percepts in everyday life; and neuronal mechanism of nonveridical perceptions.