Calls for solidarity are always accompanied by feelings of urgency. This is true both for the multifarious practical expressions of solidarity and for the intellectual usages of the concept. At the outbreak of the war in Iraq, for instance, the word and concept of 'solidarity' were brought to the fore by three of the most influential thinkers of our time, Jurgen Habermas, Jacques Derrida and Richard Rorty. However, solidarity is still used as a mere buzzword by a surprisingly broad spectrum of the political world. The book has a twofold central objective: it aims both at proposing a variety of sophisticated historical and theoretical reconceptualisations of solidarity and at exposing and spelling out the practical implications of contemporary expressions of solidarity. These two objectives are tightly related by their common frame of reference: European societies and, possibly in the future, a European polity. Thus, a first, historical and theoretical part explores the emergence, consolidation and challenging of the concept of solidarity in the context of differences in social, religious and political conditions within Europe. A second, more 'empirical' part investigates the most crucial challenges posed to solidarity in the European space: the EU integration process itself, immigration, Islam, the relation of Western Europe to Eastern Europe, and to developing countries.