The aim of Developmental Science is to understand the complex interacting biopsychosocial mechanisms in the development of living organisms. Thus, Developmental Science has roots in both the biological and social disciplines and can bee seen as a meta-theory rooted in developmental principles to guide work and thinking on biology and social behaviour and their interactions over ontogeny. In this article, we depict the entwined core-elements of Developmental Science which are all more or less subject to two dimensions: a biopsychosocial and a developmental perspective. Within a biopsychosocial frame, we argue, that development is resulting of a nonlinear combination of various factors, a process that is not restricted to an organism but happens within a complex organism-environment system. On the base of the Transactional Model of Development we argue, that it is necessary to combine a biopsychosocial view of development, considering the reciprocal relationship of environment, phenotype and genotype, with an interactionistic view of human development over time. Some of the fundamental assumptions of the Transactional Model of Development can be assigned to the so called Nature-Nurture debate and lead us from predetermined to probabilistic epigenesis. Next, we give examples to explain how genes and gene products lead to behaviour, introducing Gottlieb's (1992) model of ontogenetic development that depicts the completely bidirectional nature of genetic, neural behaviour, and environmental influences over the course of development. Finally, we refer to models of self-organization to clarify that ontogenetic development is a constructive process of qualitative reorganization within and between systems.