Traditional psychiatric textbooks describe schizophrenia as a clinical enigma of unknown aetiology. However, this is no longer true. We now know a great deal about the risk factors, or contributory causes, of schizophrenia. These can be roughly divided into two main types; those which result in a) aberrant neurodevelopment and b) those which cause dopamine dysregulation; both characteristic abnormalities found in schizophrenia. Genetic factors are, of course, pre-eminent. These will be discussed elsewhere. However, certain environmental factors have been consistently associated with schizophrenia. Some such as adverse obstetric events (e.g. prenatal infection, perinatal hypoxia) impair neurodevelopment. Others such as abuse of drugs such as amphetamines, cocaine and cannabis which increase striatal dopamine also increase risk. In recent years it has become clear that heavy use of high potency cannabis is responsible for a significant proportion of psychosis. Psychotogenic "legal highs" such as synthetic cannabinoids and cathinones are becoming an increasing cause of acute psychosis. A range of social adversities such as child abuse, adverse life events, migration/minority ethnicity appear also to facilitate dopamine dysregulation and consequent psychosis. Curiously, psychosis is more common in those born and brought up in large cities than in rural areas. Most recently, it has become clear that the incidence is much higher in cities in Northern countries such as UK and Holland than in both rural and urban areas in Spain and Italy. The exact reason(s) for these differences are unclear but speculation centres on social fragmentation and social isolation. The challenge for researchers is now to trace the pathogenic pathways from risk factors to psychosis.