This chapter explores the role of ideologies (i.e., socially shared belief systems) to justify or challenge existing social systems. The chapter begins by defining ideologies and examining their origins, focusing on how they are shaped by socialisation, collective identities, and power relations. It then discusses system-justifying ideologies, such as right-wing authoritarianism, social dominance orientation and belief in a just world, which legitimise inequality and attribute privilege to merit. In contrast, the chapter also explores system-change ideologies that challenge inequality and promote social change, focusing on the psychological processes that lead advantaged group members to embrace these ideologies and become allies. Psychological dynamics underlying allyship and solidarity are also discussed. In sum, the chapter highlights the complexities of allyship, noting that motivations can vary and that intersectional privileges can either foster or undermine solidarity, depending on shared identities and norms within groups.