Over the past few years a rapidly growing number of psychological and biological disciplines have been converging on the centrality of emotional processes in human development. Indeed these interdisciplinary studies are elucidating some of the fundamental psychobiological mechanisms that underlie the process of development itself. Contemporary developmental psychology, through attachment theory, is now focusing on the early ontogeny of adaptive socio-emotional functions in the first years of life. In current thinking development is ‘transactional’, and is represented as a continuing dialectic between the maturing organism and the changing environment. This dialectic is embedded in the infant-maternal relationship, and affect is what is transacted in these interactions. This very efficient system of emotional exchanges is entirely non-verbal, and it continues throughout life as the intuitively felt affective communications that occur within intimate relationships. Human development cannot be understood apart from this affecttransacting relationship. Indeed, it now appears that the development of the capacity to experience, communicate, and regulate emotions may be the key event of human infancy.