Psychosis is one of the most severe and distressing mental illnesses. This present thesis is an attempt to try to understand what a psychosis is and what can cause it when seen both from a psychodynamic and a cognitive theoretical point of view; schizophrenic psychosis in particular. I it presented that a psychosis is a state of mind where a person’s perception of the reality is severely disturbed by hallucinations, delusions and a disorganized behaviour. Thus a psychotic person perceives and experience things that only exist in the person’s subjective reality. The lack of reality testing is also found in schizophrenia which is a psychotic state influenced by disturbed contact with the surrounding world. Theories presented by Klein, Winnicott and the cognitive theory are discussed using a case as a mutual starting-point and focusing on the things that are characteristic for a psychosis.