The comparison of societies is an ordinary activity, a practice that can be observed far beyond its scholarly uses. Based on an ethnographic study of advocating for the right to education of undocumented children in Quebec, this article studies how and why activists compare societies. This study shows the crucial role played by comparison in constructing the public issue of the schooling of undocumented children in Quebec. Whether it is to raise public awareness, to define and highlight the problem, to put together action plans, or to develop convincing arguments, the reference to foreign experiences (in particular French, American, and Ontarian ones) is key. However, activists who compare societies encounter similar issues to those that social science researchers come across. Comparison as a political activity is therefore at the heart of processes of diffusion and circulation of social problems from one social and national space to another.