The principle of solidarity is particularly important now because it is in juxtaposition to some current self-centered trends in politics: the crises that have upset the world in recent years, such as migrations, hegemonic aspirations, pandemics, and wars, have made self-evident the inadequacy of such selfish politics. It therefore seems very useful to understand the role that solidarity could play in contemporary scenarios. This book thus collects various contributions on the principle of solidarity in international law. Firstly, it reconstructs the foundations of solidarity in law and investigates the origins of the principle. Subsequently, it tries to ascertain if solidarity exists as a principle in international law and, if so, what its scope is. The book then examines the eventual implementation of the principle of solidarity in regional organizations law: the question is whether solidarity is more effective and if it is actually better fulfilled when relationships between states get stronger, such as in regional and sub-regional organizations. Such implementation is notable in EU Law, but the analysis also involves other regional and sub-regional organizations, namely the African Union, ECOWAS, the League of Arab States, and MERCOSUR. This book takes into account not only some distinctive areas of solidarity, such as migration law, or specific institutional contexts where solidarity is a declared principle, objective or value, for example, the European Union, but it also considers whether, and to what extent, the manifestations of solidarity differ in disparate settings, trying to discover the reasons behind such divergences. The book will be of interest to researchers and academics in the areas of International Law, International Organizations Law, European Law, Human Rights Law, Business Law, and Constitutional Law.