This chapter introduces the concept of infant behaviour and temperament development alongside current definitions of parent-infant bonding—e.g. the “emotional, behavioural, cognitive and neurobiological tie of the parent to the child” (Nakić Radoš et al., J Reprod Infant Psychol 42(2):142–65, 2024). Early infant behaviour can be viewed as an expression of the baby’s physiological and neurobiological wellbeing and is the primary way that babies communicate with their parents or caregivers (Brazelton and Nugent, Neonatal behavioral assessment scale. Cambridge University Press, 1995). It is increasingly becoming known that the mother and baby’s experience of childbirth could affect them both in terms of short- and longer-term wellbeing, behaviour, mood and confidence. We must learn how to listen to and understand baby behaviour so that we can adjust childbirth and postnatal care to make it less overwhelming for the neonate, especially if they experience complications or are born preterm. These situations can be equally overwhelming for parents, who may then struggle to bond with their baby. The chapter provides a summary of the Bucharest Early Intervention Project and what we can learn from this about the importance of supporting the early building of strong, secure parent-infant relationships.