Studies on human patients and animal models of disease have shown that disruptions in prenatal brain development are a root cause of mental diseases. Since the proper brain development is achieved by strict spatiotemporal control of brain regionalization, neurogenesis, cell migration and synaptogenesis, anomalies in one or more of these events during embryogenesis can lead to predisposition to suffer a mental dysfunction during postnatal life. Moreover, although it is widely accepted that proper cognitive function in humans requires appropriate interactions with one's environment, evidence has shown that susceptibility to develop psychotic symptoms is largely specified by a genetic component. Here we will review some molecular and cellular mechanisms of brain development that have been related to the predisposition and development of bipolar disorder and schizophrenia.