It is difficult to think of any behavioral process that is more intrinsically important to human beings than social attachment. Feeding, sleeping and locomotion are all necessary for survival, but humans are 'a social animal' and it is our social attachment that we live for. One of the early pioneers in this area, Harry Harlow, described the different behavioral processes that are involved in the formation of parent-infant, filial and pair(male-female) bonds. Each of these involves multi-sensory processing and complex motor responses. Over the past decades, studies in a range of vertebrates, including humans, have begun to address the neural basis of attachment at a molecular, cellular and systemic level. This review describes some of important insights from these works, involving three different areas:1) Neurobiological research of infant-parent, parent-infant attachment, 2) Animal studies regarding attachment, 3) Neurobehavioral studies of maltreatment/deprivation causing serious breakdown of attachment relationship in humans.