What is the role of solidarity in current legal systems? What are the challenges of Law in modern societies? Solidarity, as a legal principle, has been studied as a value of the Social Rule of Law model, however, we can expand its role as an essential principle of Law. The foregoing allows us to respond to the challenges presented by the Law before the reality of 21st-century societies, those that present high degrees of inequality, new ways of building social relations, and a marked tendency towards individualism. Because of this, it is important to analyze and reconsider the basic concepts and fundamental pillars of Law, so that it can face the challenges posed in an appropriate manner.