Adult attachment can be defined as a stable tendency in making efforts to obtain and maintain physical proximity and emotional closeness with a specific person who provides one with safety and security. This disposition is regulated by attachment internal working models, which are cognitive-affective-emotional drafts built on the individual experiences of interpersonal relationships. British psychologist John Bowlby, who conceived the attachment theory, defined attachment as a “lasting psychological connectedness between human beings.” It is a deep and enduring emotional bond, not necessarily mutual, that connects one person to another across time and space. In this entry, the general importance of adult attachment and the development and physiological function of attachment throughout the lifespan are described.