Infant attachment is one of the key factors for child mental health. Whereas secure attachment predicts social competence and resilience, insecure and disorganized attachment are risk factors for later mental problems. These long-term effects can be explained by the importance of early attachment for children’s stress-regulation capacities. Securely attached children are confident that the parent is available when needed, which provides them with an effective dyadic stress-regulation strategy: they can approach the parent for comfort and help in stressful situations. In insecure and disorganized children, effective dyadic stress-regulation strategies are undermined, because the child cannot rely on the parent for help. Dyadic stress-regulation strategies form the basis for later self-regulation capacities, which in turn play a major role for mental health.Despite this importance, the origins of individual differences in attachment quality are still not fully understood. Questions remain about the development of attachment relationships in the first year of life, and about which factors determine individual differences in attachment quality. Research has mainly focused on parental behavior, which, however, only explains part of the variation in attachment quality. This lack of understanding the underlying processes of attachment formation is termed the “transmission-gap”.Therefore, the overall goal of the proposed research project is a better understanding of the mechanisms of infant attachment formation and subsequent stress-regulation capacities by studying:1) The “undiscovered” origins of attachment in the first year of life2) The “undiscovered” role of the infant in shaping early attachment relationship3) The role of early bi-directional processes between infant and mother for the development of stress- regulation capacities4) The role of maternal postnatal depression in these processes to identify potential adaptive and maladaptive pathways.