This chapter outlines key concepts and contexts in relation to public participation. Few areas of international law have developed so rapidly as that on public participation in environmental matters. With some exceptions, this notion hardly existed in international law until the early 1990s. The development since then stems from two disciplines of international law: environmental law and human rights law. It pertains to public participation in decision-making at the national as well as international level, and involves important elements of multilevel governance. One important feature for this legal development, and also for the continuing vitality of the discourse, is the possibility in some environmental and human rights regimes for members of the public to access independent international review mechanisms. These bodies take the form of courts, committees, or commissions, with the mandate of examining whether state parties live up to their international obligations of ensuring participatory rights in their national jurisdictions.