This chapter is devoted to the pathways that link the peer relations of children with neurodevelopmental disorders to their mental health. Many, if not most, of the studies conducted so far are correlational, which means that peer relations and mental health occur together in some way that cannot be specified. Conceptual models emerging largely from the discipline of developmental psychopathology suggest ways of tracing the pathways of influence more conclusively. One possibility is that impaired peer relations are an inherent aspect of the disorder that is almost always present. This is implied in the contemporary conceptualization of autism spectrum disorder. Another possibility is that impaired peer relations are a marker, a feature so frequent that it can be used with some reliability to identify the disorder. Mediated models indicate that there is some intervening variable—that impaired peer relations cause some other disturbance (e.g., depression, anxiety, missed opportunities to learn social skills) that, in turn, leads to psychological disorder. Moderated models are based on the hypothesis that peer relations play a protective role. For example, individuals who experience problematic peer relations are less likely to develop a form of psychological disorder if their peer relations are rewarding and appropriate. Achieving the conceptual clarity provided by moving beyond one-shot correlations often requires longitudinal data collection. Such clarity also depends on using sound measures of peer relations, psychopathology, and any intervening variables.