In this chapter, we discuss theoretical and conceptual models that use an organismic or systems metaphor for understanding families. We suggest that such theories are important for stimulating new research and organizing existing data, and that advances in these theories over the past few decades have expanded the potential for understanding child development, as well as adult adaptation and the development of close relationships. These paradigms follow from models that view development as resulting from the transactional regulatory processes of dynamic systems. Such models are helpful for considering multiple influences on development and adaptation and have implications for the design of effective interventions. We focus on the specifics of systems theories as applied to families, and the research generated by or consistent with these views. Our review is not exhaustive; rather, we intend to give a sense of the direction of this work and its importance for the understanding of development and adaptation.