Hallucinations are a psychopathological symptom occurring in mental disorders of various etiology. Hallucinations are defined as perceptions occurring in absence of any external stimulus. Although they are often diagnosed and their phenomenological descriptions can be found in every handbook and classification, nevertheless their etiopathogenetic background is not quite clear. In the paper some lines of research conducted in the past or recently are presented. One approach is to seek neuroanatomical site of damage in the c.n.s., and biochemical changes resulting in occurrence of hallucinations. In another approach explanation is sought on the grounds of psychophysiological theories. Each of these research approaches provides evidence on the complexity and non-homogeneity of the hallucinations phenomenon. A question arises whether auditory hallucinations are the same phenomenon as hallucinations in other sense modalities, and whether it is possible to produce a single etiopathogenetic concept accounting for this psychopathological phenomenon as a whole.