While it is often assumed that behavioural development must be based upon both physical law and the biological principles of morphogenesis and selection, forging a link between these disciplines has remained an elusive goal. This book addresses the question of how familiar human functional acts - eating, walking, manipulating objects, smiling, etc. - emerge during infancy due to both intrinsic dynamics and selective processes. The central thesis of the book is that during perceptually guided spontaneous activity, a variety of biodynamic devices for doing different kinds of work are assembled and adapted to specific tasks. Following the introductory chapters, which explore principles from the fields of dynamics and ecological psychology, the author introduces a theory of the development of action systems based upon both self-organisation in complex systems and perceptually guided selective processes. The theory is then examined in the context of development of each of several action systems. The book addresses many long-standing issues in behavioural development, including the apparent disappearance of so-called primitive behaviours, the emergence of new skills, and the role of the caregiver on skill acquisition. The prospects for extending the theory to atypical development and to other domains such as cognition and language are also considered.