This article reviews how ideas from 20th century physics and biology have come to play important roles in the study of development and how these ideas have informed a relatively new paradigm in developmental science: relational developmental systems, a synthesis of developmental biology and developmental psychology. Employing concepts such as emergence and self-organization, epigenetics and epigenesis, and an ontological framework that stresses levels of increasing organization and complexity, the relational developmental systems paradigm embraces a thoroughly holistic and nonreductionistic account of development and behavioral origins. It furthermore promotes psychology as a unique science, irreducible to biological science—though genes, brains, and other biological processes are factors that participate in the developmental process, such factors do not cause behavioral origins and are only explicable in the context of the developing system as a whole.