This essay explores implications of current trends in developmental science for understanding psychopathology at the dawn of the new millennium. Over the past half century, it has become clear that uniform and general principles of development (i.e., those that are applicable at all times, to all people, and in all places) will be of limited utility in understanding the processes of greatest interest in development and psychopathology. Instead, such processes are characterized by complexly organized individuals engaged in developmental transactions within multiple contexts (ranging from the biological environment of neurons to the cultural systems of meaning that shape people's lives). These transactions in turn often yield variable outcomes. In order to portray how we have come to this conclusion, we first provide a view of contemporary research in three areas of early development: the biology of the developing brain, the complexities of early emotional development, and the cultural contexts of child development. We then trace how an increasing appreciation of organized complexity, developmental transactions, and the meaning of context have played out in the emerging field of infant mental health before closing with our vision of new opportunities for the study of experience in the midst of variation.