This volume presents a high-level scholarly discussion on whether the concept of solidarity functions as a structural principle of international law and to what extent it has become a full-fledged legal principle. Each contributor addresses these questions by examining normative operations of the principle of solidarity in different branches of international law - including international disaster law, international humanitarian law, the law of development cooperation and international environmental law - as well as the relationship between the principle of solidarity and other legal principles such as the responsibility to protect and intergenerational equity.