Visual hallucinations have played an important role in religion, culture, and all concepts of mental disease. A brief review of these phenomena in healthy individuals as well as in clinical medicine is provided. We analyse the pathomechanisms involved in the development of visual hallucinatory experiences and argue that no single model can serve to explain all the phenomena encountered in the field of visual hallucinations. We question the validity of the current distinction between hallucination and illusion, and delineate conditions which are only appreciable psychologically, e.g. the 'imaginary playmates' of childhood and visual hallucinations in the face of a severe grief reaction. At least three putative mechanisms for the genesis of visual hallucinations can be described in biological terminology: irritative phenomena, release phenomena, and processing disturbances within the visual pathways.