Age structures our lives and societies. It shapes social institutions, roles, and relationships, as well as how we assign obligations and entitlements within them. Each life-stage also brings its characteristic opportunities and vulnerabilities, which spawn multidimensional inequalities between young and old. How should we respond to these age-related inequalities? Are they unfair in the same way gender or racial inequalities are? Or is there something distinctive about age that mitigates ethical concern? Justice Across Ages addresses these and related questions, offering an ambitious theory of justice between age groups. Written at the intersection of philosophy and public policy, the book sets forth ethical principles to guide a fair distribution of goods like jobs, healthcare, income, and political power among persons at different stages of their life. At a time where young people are starkly underrepresented in legislatures and subject to disproportionally high unemployment rates, the book moves from foundational theory to the specific policy reforms needed today. If we are ever to live in a society where people are treated as equals, the book argues, we must pay vigilant attention to how age membership can alter our social standing. We should regard with suspicion commonplace forms of age-based social hierarchy, such as the political marginalization of teenagers and young adults, the infantilization of young adults and older citizens, and the spatial segregation of elderly persons. This position carries important implications for how we should think about the political and moral value of equality, design our social and political institutions, and conduct ourselves in a range of contexts including families, workplaces, and schools.