Social identity research was pioneered as a distinctive theoretical approach to the analysis of intergroup relations but over the last two decades it has increasingly been used to shed light on applied issues. One early application of insights from social identity and self-categorization theories was to the organizational domain (with a particular focus on leadership), but more recently there has been a surge of interest in applications to the realm of health and clinical topics. This article charts the development of this Applied Social Identity Approach, and abstracts five core lessons from the research that has taken this forward. (1) Groups and social identities matter because they have a critical role to play in organizational and health outcomes. (2) Self-categorizations matter because it is people's self-understandings in a given context that shape their psychology and behaviour. (3) The power of groups is unlocked by working with social identities not across or against them. (4) Social identities need to be made to matter in deed not just in word. (5) Psychological intervention is always political because it always involves some form of social identity management. Programmes that seek to incorporate these principles are reviewed and important challenges and opportunities for the future are identified.