Hallucinations occur when people have perceptual experiences (i.e. see things, hear things, smell things, feel things, and sometimes taste things) in the absense of an external stimulus (i.e. when there is nothing there. These experiences feel very real to the person having them, even though they are not. Some types of hallucinatons are very common and are not indicative of any mental disorder. For example, many people report seeing things right before the fall asleep or right after they wake up. However, some types of hallucinatons might be indicative of a more serious type of disorder, such as schizophrenia. Hallucinations might also be caused by various substances.
