Attachment is a fundamental need, comparable to hunger or sex, that even infants form with an adult caregiver. When young children are separated from their caregiver for extended periods, they suffer severe psychological damage: from chronic sadness and depression to complete apathy, from aggression to delinquency. In his foundational work, John Bowlby presents his theory in detail. Each volume is dedicated to a specific thematic focus. Bowlby observed children who were separated from their parents after World War II, for example during hospital stays, or those who grew up in orphanages without a stable caregiver. From these research findings, he derived therapeutic measures to support the children's development.