This chapter recognises that our capacity to define the self in group-based terms has profound implications for both psychology and behaviour. It explains what these implications are – with a view to explaining why social identity is such an important concept for human health. The process of depersonalisation describes how people come to see themselves in terms of social identity, but it does not explain when or why this occurs. The first psychological resource that is derived from social identity pertains to perceptions of social connection – the sense that one is psychologically close to, and yoked with, other people. However, as group members who recognise that they share social identity, their similarity and interconnectedness become much more apparent and much more significant. A key reason for this is that a sense of shared social identity transforms others who are different from the self into fellow ingroup members who are part of the self.