Solidarity, as a legal principle, is a new and evolving concept, whose nature and functions in constitutional adjudication remain unclear. This article investigates whether solidarity between the Member States has the legal quality of a general principle, and if so, what its meaning is within the EU legal order. Based on an examination of Treaty provisions concerning solidarity between the Member States in different areas of EU law and the jurisprudence of the Court of Justice on solidarity, it concludes that solidarity between the Member States is, in fact, a general principle of EU law. However, the meaning ascribed to the principle in case law is inconsistent. The article argues that, in order to qualify as a general principle, solidarity must have a narrower meaning than proposed in particular by the General Court in a recent case on energy solidarity. It defines what solidarity involves if it is conceptualised as a general principle.