Solidarity is a complex social phenomenon, a multifaceted term used in a variety of meanings and situations. Social solidarity, transformed to a legal principle, lies at the foundations of all European social security systems. This article explores how the principle of solidarity is understood in EU law, especially in the case-law of the Court of Justice of the EU. Delineation between solidarity as a fundamental value and solidarity as a legal principle is not always clear-cut in the EU law. However, the content and impact of EU law increasingly dictates the perception of solidarity and evolution of the principle of solidarity in national social security systems. The aim of this paper is to show how the impact of EU law is liable to reshape and redefine social security systems throughout the EU.