This unit first described the character of international environmental law as it responds to and tries to deal with increasingly critical environmental problems. The multifaceted nature of the physical, social and cultural problems that pose growing stresses on our environment compels international environmental law to be encompassing and inherently multidisciplinary. The evolution of international environmental law has been driven by the increase in the number and range of environmental problems over the past century. Its development can be distinguished according to four periods: the period prior to 1945; the period after the Second World War in 1945 to the Stockholm Declaration of 1972; the post-Stockholm period to the Rio Declaration in 1992; the post-Rio period to the World Summit on Sustainable Development in Johannesburg in 2002 and to the present. The latter of these phases is notable for the increasing emphasis on sustainable development. This emphasis especially incorporates the needs of developing nations in terms of their economic development. Economics considerations have become a central driving force in the continuing evolution of international environmental law. This evolution recognises the disparate histories, interests, and economic and social capacities of developing versus developed countries. The complexity of the demands which international environmental law is endeavouring to meet compels an understanding of its sources. As elucidated in the Statute of the International Court of Justice, there are four sources of Public International Law: treaties, international custom, general principles of law, and subsidiary sources including decisions of courts and tribunals and the writings of eminent jurists.