In discussions about the Greek debt crisis, the refugee crisis, and EU regional policy, appeals to solidarity are often made. European Union citizens and member states are asked to be solidary with Greece, with member states visited by large numbers of refugees, and with regions burdened instead of benefitted by the internal market. It is not so clear, however, what this appeal to solidarity is based on or in what way it differs from (and relates to) demands based on justice. This paper argues that normative theories of EU solidarity usually do not discriminate systematically between solidarity and justice or at least do not problematise the way in which these standards are related. The result is that EU crises cannot be analysed properly. In this paper, I explain why Kantian legal thought (Kant in The metaphysics of morals (revised edition). Cambridge University Press, Cambridge, 2017; Ripstein in Force and freedom: Kant’s legal and political philosophy, Harvard University Press, Cambridge, 2009; Rawls 1993) helps us understand the underlying issues more clearly.