Presents a theoretical model to understand and assess individual infant development. The model focuses on the dynamic, continuous interplay of various subsystems within the organism: the autonomic, the motor, the state organizational, the attentional-interactive, and the self-regulatory. The organism forges ahead, negotiating emerging developmental agenda while simultaneously seeking to attain a new level of modulated, functional competence. Developmentally salient aspects of the environment are actively sought as fuel in this process. This synactive model of development promises to be helpful in identifying specific ingredients of the early developmental process and in structuring specific supports for preventive and ameliorative work when difficulties in differentiation and regulation are identified. A systematic assessment procedure to identify difficult areas of modulation integration is briefly described, and examples of environmental structuring to maintain maximal development and to reduce developmental defense and sparsity are given. (28 ref) (PsycINFO Database Record (c) 2012 APA, all rights reserved)