In this chapter the occurrence of schizophrenia, other psychotic disorders, psychotic symptoms in intellectual disability, and autism spectrum disorder is considered, as are the similarities in clinical features between these disorders. Also reviewed are the evidence for genetic overlaps between these disorders and their convergence on specific biological pathways. Finally, the evidence for overlap in environmental risk factors between these disorders is considered. Taken together these findings suggest that schizophrenia and the other functional psychoses may be related aetiologically and pathogenically with the group of neurodevelopmental disorders to which intellectual disability and autism belong. Indeed these disorders might be better conceptualized as lying on a spectrum of clinical outcomes that result from disruption to the developing brain induced by genetic and environmental factors. The chapter concludes with a discussion of the clinical implications of these findings.