The concept of schizophrenia is that of a group of illnesses with differing clinical pictures and evolution. The "modes of entry" into the illness can be dramatic, with the unveiling of an acute psychotic episode, just as they can be insidious, with a progressive withdrawal accompanied or not by prolonged disturbances of mood and possible drug experimentation that act to mask the symptoms. Psychological, pharmacological, genetic and neuro-developmental studies have led to different hypotheses of pathogenesis, none of which alone is sufficient to explain the observed troubles, nor can be formally excluded. Schizophrenia is a multi-factorial illness and the current tendency is to describe a "vulnerability" to schizophrenia with a clinical picture and evolution that is simultaneously dependant on the environmental conditions and the therapeutic modalities put in place during the management of these patients.