This chapter describes the concept of attachment as an organizer in the development of children, which has an impact throughout life. The origins of the theory with John Bowlby and Mary Ainsworth are highlighted, as well as further research on attachment styles, disorganized attachment and parent infant relationships in general. The concept of attachment was based on studies in ethology and further developed to study the effects of experiences in humans. The reality of what happens to the child during the early years has an impact on “working models of relationships” and whether the person feels secure and able to explore the world. The impact of separations, losses and mourning in children are also described, as well as the fear of separation and being all alone as another important organizer in the development of the mind. Finally, we discuss the implications of attachment to understand difficulties like anxiety, personality disorders and chronic suicidality. We briefly describe some of the strategies developed to assess the attachment style of a person during infancy, adolescence and adulthood.