An understanding of developmental phenomena demands a relational or coactive concept of causality, as opposed to a conceptualization that assumes that singular causes can act in isolation. In this article we present a developmental psychobiological systems view of relational (bidirectional, coactional) causality, in which it is proposed that developmental outcomes are a consequence of at least two specific components of coaction from the same or different levels of a developmental system. The levels are genetic, neural, behavioral, and environmental; the latter level includes the cultural, social, and physical aspects of an organism's environment. We show the applicability of this view to the understanding of the development of normal and abnormal behavioral and psychological phenotypes through illustrations from the existing animal and human literature. Finally, we discuss future possibilities and potential stumbling blocks in the implementation of a more fully realized bidirectional, coactional perspective in developmental psychopathological research.