Solidarity is a widely debated and researched subject. However, scholarly writing has generated little evidence on the scope and structure of citizens’ solidarity within Europe, particularly in regard to cross-national solidarity between citizens and organized civil societies. Chapter 1 introduces this topic and provides an integrated account of available scientific knowledge. Moreover, it proposes a conceptual framework of analysis that aims to do justice to the multidimensionality of solidarity. It is argued that solidarity implies consensual and conflictive, social and political, attitudinal and behavioural dimensions. Additionally, it stresses the need to respect the multilayered structure of solidarity, given that solidarity is organised and institutionalised at the level of interpersonal networks, organisational fields, welfare states and public discourses. Finally, it stresses the need to consider the specificities of European solidarity, given that European solidarity is organised and institutionalised to a different extent at these three levels, when compared to solidarity within a national context.