This chapter introduces the idea of a relationship between attachment processes and the development of the capacity to envision mental states in self and others—the capacity that is referred to in this book as mentalization or reflective function. Throughout this book, we develop the argument that the capacity to mentalize is a key determinant of self-organization and affect regulation, and we maintain that this capacity is acquired in the context of the child's early social relationships. Here we give an overview of the evidence for an association between the quality of attachment relationship and reflective function in the parent and the child. We offer some hypotheses about the development of reflection in the context of the infant-caregiver relationship. We then interpret these data and speculations in the context of current models of theory-of-mind development.