Friendships are a primate speciality, and have evolved to buffer us against the stresses of living in large social groups. They have a bigger effect on our psychological health and wellbeing, as well as our physical health and wellbeing, than anything else. Friendships are, however, extremely expensive to create and to maintain, both in terms of their time cost and in terms of their underpinning neurobiology. In this lecture, Ill explore the behavioural, cognitive and neurobiological bases of friendships, and show how we use these as a basis for forming mega-communities.