Attachment is an important developmental domain and is known to have an impact on other domains throughout life as well, like mental and physical health, academic achievement and social functioning (Cassidy, & Shaver, 2016). Attachment theory (Bowlby, 1969) points at the importance of effective care during stress to develop a secure attachment and build trust. This means that whenever a child is distressed, it can count on its caregiver to provide help and support. Bowlby suggested that children develop trust and secure attachment representations when they have multiple experiences of receiving help and support when needed. Trust and secure attachment subsequently stimulate care seeking behavior. However, when care is lacking or inconsistent, children are more likely to develop an insecure attachment to the caregiver. As a consequence, they fail to adequately turn to their caregivers for help during distress. Instead they develop more anxious (driven by fear for rejection) or more avoidant (driven by the desire to avoid support seeking) insecure attachment styles. According to Bowlby, these experiences become internalized in Internal Working Models. These are cognitive representational models in which experiences during care-related interactions with caregivers are stored. These Internal Working Models which at least partly consist of Secure Base Scripts (Waters & Waters, 2006) are mental scenarios of a child in distress seeking proximity to an attachment figure, followed by help and ending in a feeling of being back on track. These Secure Base Scripts shape expectancies for future interactions and guide the individual’s behavior. It is not well known however, how these Secure Base Scripts are formed. The current project builds on the assumption that they develop through safety conditioning processes (Quirk & Milad 2012). In learning theory, the probability of the co-occurrence of two stimuli, here care (unconditioned stimulus) together with the attachment figure (conditioned stimulus) in a context of stress, is called contingency (Rescorla, 1966; De Houwer & Beckers, 2010). If the attachment figure is repeatedly paired with care, it acquires a helpful meaning and signals safety. Contingency thus is the probability that the attachment figure provides support during stress in the child. Through repeated experiences of receiving care from an attachment figure during distress (high contingency) the attachment figure gets associated with care-related experiences that reflect changes at the level of endocrinological functioning (e.g., cortisol decrease, oxytocin increase; Hostinar, Johnson, & Gunnar, 2015). As a result, the attachment figure becomes a predictor that the stress will be reduced by provided care. Although there seems to be stability in the Secure Base Script over time, they still can change or become more complex based on additional experiences between early childhood and adolescence. During middle childhood, biological processes enhance brain capacity to organize experiences during social interactions in a cognitive script-like fashion (Crittenden, 2000). Therefore, middle childhood is a suitable period to study the development of attachment. When investigating the development of attachment, research has focused mainly on social influences such as parenting. However, little research has been done on the underlying endocrinological processes. In this project we will focus specifically on oxytocin. Oxytocin has previously been suggested to play a role in the development of attachment (McQuaid, et al., 2016). Oxytocin is a hormone and neurotransmitter that is produced in the hypothalamus and released from the posterior pituitary gland when a child encounters an interaction with an attachment figure when the attachment bond is described as secure. When a child is in distress and searches for proximity to his attachment figure, and that attachment figure then provides help and support, oxytocin is released and causes a feeling of warmth and security (Smith, & Wang, 2012). In conclusion, we will investigate whether contingency, oxytocin, and cortisol explain variability in the development of (in)secure attachment styles, trust, and the Secure Base Script. We predict that higher levels of contingency during daily interactions between the child and the attachment figure, increases in oxytocin, and decreases in cortisol during care at single learning experiences play a role in the development of attachment over time. This study will be carried out at the Center of Clinical Psychology (Faculty of Psychology and Pedagogical Sciences at the KU Leuven) and is funded by the FWO (G075718N).