This chapter addresses the question of the normative and legal folds of sustainable development. By initially providing theoretical underpinnings on the concepts of soft law, rules and principles, it is concluded that sustainable development has a principled nature, providing arguments for its addressees to move in a certain direction. The Chapter then moves to the theory of the sources of international law, to inquire on the validity of sustainable development. Sources listed in Article 38 of the ICJ Statute as well as General Assembly Resolutions are discussed theoretically and then contextualized in relation to sustainable development. The subsidiary means of jurisprudence is also explored as a pointer to sustainable development being nested in one of these sources. The conclusion that is drawn, is that sustainable development is a customary principle of international law.