The studies in the first part of this thesis explored the association between peer environmental influences and the stress system. In chapter 2, we tested the association of peer social experiences within the early elementary school classroom with individual differences in cortisol concentration across a day. In chapter 3, we explored acute stress responses to a psychosocial stress task among elementary school children, who either had a history of prolonged negative or positive peer experiences in the earlier years of elementary school. A psychosocial stress task induces stress during a laboratory procedure, in our case through a conversation with unknown peers, who gradually exclude the child. In Chapter 4, we explored whether warmth in the teacher-student relationship influences the association of (lower) peer acceptance among classroom peers with daytime cortisol levels. Warmth was measured by asking children about their experiences of teacher support. Finally, in Chapter 5 we further dove into the role of the broader context, i.e., living in a war-conflict area, on the link between peer relations, (perceived) stress activity and internalizing (anxiety) symptoms. Do peer relationships go under the skin? Overall, the results of Chapter 2 and Chapter 3 suggest that already in the elementary school period, adverse social experiences like low peer preference get “under the skin”. Both studies emphasized that peer relationships are an important factor in the development of children’s stress (re)activity. The combined results of the two studies thus suggest that already in late elementary school period, adverse social experiences like low peer preference affect the stress system (re)activity of children. Our findings suggest that low peer preference represents a social stressor that impacts both major stress systems as early as during childhood. As a response to social stressors, stress system activity is altered, which comes at costs. Altered stress system (re)activity in low peer preferred children is likely linked to less well- developed self-regulation and behavioural problems, although this needs to be further examined. What is the role of the context in the link between negative social stressors and (perceived) stress system activation? Overall, the results in chapters 4 and 5 suggest that a discrepancy in social support in the micro-environment (classroom setting; chapter 4) or experienced distress in the macro-environment (living in a war-conflict region; chapter 5) can augment the impact of (perceived) stress from other social sources. Children’s capacity to effectively cope with daily stressors may be diminished in children living in a war-conflict region. Consequently, both the immediate classroom environment and the environment in which children live, increases risks for developing anxiety symptoms. Negative peer relations go skin deep – Implications Our findings suggest that children with positive peer experiences have a healthy, adaptive stress (re)activity functioning. Low peer preferred children may show altered stress system activity, which might contribute to the inability to respond in an adaptive way, both in terms of behaviour and emotions in response to daily stress. Similarly, heightened stress activation levels and altered stress system responses have been linked to anxiety symptoms. Thus, classroom dynamics bear the risk for developmental problems, including the development of anxiety symptoms. Social experiences in the peer context and the broader environmental context play a key role in the development of children’s stress system activity and could further shape their emotional and behavioural outcomes throughout development.