The aim of this chapter is to present the most important discoveries from developmental research over the last twenty years that in our view pertain directly to the psychoanalytic understanding of self-development and have clear clinical implications for the treatment of individuals whose primary disturbance is rooted in disorganized self-representation and affect dysregulation. We wish to distinguish the model of self-development and affect regulation proposed in this volume from other contemporary developmentally grounded psychoanalytic views. We trace the development of mentalization from infancy and engage in the developmental debate about exactly when the intentional stance is achieved. In our view many recent contributions have overstated the case for early intersubjective processes in self-development. We argue that intersubjectivity is an emergent phenomenon whose establishment is a function of early interactive processes within an attachment context. In chapter 3 we laid the foundations for a model of self-development rooted in interpersonal understanding; in this chapter this model is elaborated, and five stages in the development of the self as agent are distinguished. The chapter also lays the crucial groundwork for the explorations of borderline personality disorder in terms of an early nonmentalistic perception of causality in social actions— the teleological stance. Finally, the chapter gives a relatively 204comprehensive summary of the developmental literature concerning the development of an understanding of self and others as intentional mental agents.