The international society moves towards an international community—a sociological unit where common concerns are the core element. This evolution raises the question of how solidarity has impacted the international legal framework. This chapter draws on the concept of solidarity as a core normative ideal underpinning the development of modern international law. It investigates the influence of the common interest on international solidarity and the evolution of international law. First, focusing on environmental issues, it elaborates on international solidarity as the mysterious concept driving the evolution of international law to address global challenges. Then, it analyses how solidarity has shaped the legal framework, both at the normative and institutional levels.