This essay introduces a special issue on the complex and contradictory ways in which young activists and youth organisations encountered and experienced internationalism. It argues for the need to pay greater attention to the ambiguous encounters – involving seemingly benevolent aims but also blind-spots and prejudices – that were created by transnational youth mobilities and by young people’s participation in international ventures. We first consider meanings of 'youth' within different twentieth-century contexts and comment on the transnational mobilities in which young people participated. We then outline how youth-based internationalism took different shapes, discussing its left-wing and Christian manifestations in particular, and noting how internationalism was articulated through different forms of collective action. The essay makes a case for combining perspectives from social and transnational history to demonstrate the complex character of internationalism, which different groups of young people experienced as both empowering and exclusionary.