The attachment phenomenon is increasingly the focus of many social work interventions. Biologically described, differentiated types of attachment relationships result from qualities of repetitive interpersonal brain-to-brain encounters with caregivers that affect variations in emotional/affective arousal regulation; this research takes place within the field of interpersonal neurobiology. The particular focus of this entry is implicit and explicit manifestations of certain structures and functions of the brain and nervous system critical to the bio-regulation of emotions. In-born emotional circuitry is sculpted by postnatal caregiving, resulting in a pattern of emotion regulation that leads to certain attachment types. Although there is no attachment circuit per se, emotional circuits in the low brain can work together with other parts of the brain to create various types of attachment. Neurobiological influences act on the development of attachment styles during childhood that may persist into adulthood are briefly reviewed. Attachment research and often subtle biological arousal considerations are also mentioned. Over the years since John Bowlby first began to contribute his work on attachment, research has highlighted, more and more, the various biological aspects. These include the profound biological significance of the circular relationship between separation, responses to separation, and resulting attachment templates. The roles in the attaching process of neuroception, mirror neurons, transfer of affect, and long-term potentiation are described. Selected treatment theories, primarily from the social work literature, are examined for their implicit focus on aspects of the neurobiology of attachment relationships.