This chapter explains the development of attachment relationships between children and their parents/caregivers, foster and adoptive parents and different family members, including siblings, grandparents and step-parents. It discusses the processes that lead to secure or insecure attachment, and the Strange Situation Test, and other measures of attachment. The chapter considers the impact on children of separation and loss, for example, through death or serious illness or extreme social conditions. It also considers children who have been removed from their parents for reasons of abuse or severe neglect. The chapter examines the impact of early family relationships on the development of the child’s inner world, sense of self and capacity to form meaningful bonds with others. It explores the crucial role of relationships within the family for the healthy emotional and social development of the child. The concept of the internal working model of relationships is proposed as an important explanation for mediating processes within the child and young person.