There is robust and consistent evidence from epidemiological studies showing that urban birth is associated with an increased risk of developing schizophrenia. Evidence suggests that this exposure may be associated with a sizeable proportion of cases. To date the candidate exposures underlying the urban birth risk factor have included infectious agents, low prenatal vitamin D, toxins associated with pollution, and stress. However, in general, the hypotheses proposed to explain the urban birth risk factor have been unsatisfying. In light of the general trend towards increasing urbanization, it is feasible that the attributable fraction of schizophrenia associated with urban birth may increase. The psychiatric research community should have a sense of urgency in exploring the mechanisms linking urban birth and risk of schizophrenia.