Hallucinations are false perceptions in the absence of external stimulus. They are a part of many diseases, such as schizophrenia, bipolar disorder, dementia, epilepsy and others. They can also be the result of drug abuse. During the research of hallucinations a lot of research was done on the neurotransmitters dopamine and serotonin. Dopamine has a great role in the reward system of the brain. Its level in schizophrenia is increased and this is considered the main cause of the symptoms of the disease. Dopamine level which is also increased during the consummation of several drugs (amphetamine, LSD, cocain) can induce symptoms similar to those in schizophrenia. Serotonin has a role in the regulation of mood, cognition and memory. Different disorders are linked to decreased levels of serotonin and accordingly, it is believed that the serotonergic activity in schizophrenia is decreased. The fact that LSD is a serotonin agonist and that many antipsyhotics are serotonin antagonists speaks in favor of the hypothesis that serotonin is linked to psychotic efects. This leads to the conclusion that both dopamine and serotonin are included in the proces of hallucinations. It is not known exactly why or how hallucinations occur, or why there is an increase in dopamine level. Some scientist believe that there is simply too much synthesis, while others think the synthesis is normal, but that dopamine doesn't degrade properly. In conclusion, it is considered that irregular neurotransmission due to the increased levels of dopamine is responslible for hallucinations.