We trace the development of the deep ecology movement beginning with Arne Naess’ introduction of the term in 1972. We give a detailed account of the movement comparing it to other movements for social responsibility that developed in the 20th century. We discuss Naess’ cross-cultural approach to characterizing grass-roots movements via platform principles that can be supported from a diversity of cultures, worldviews and personal philosophies. We explain Naess’ use of “ecosophy.” We describe his personal philosophy Ecosophy T, and note that some erroneously conflate it with the deep ecology movement. We present an account of his Apron Diagram that uses four levels of discourse to analyze social-political movements, which are: ultimate values in life philosophies, platform principles, policy formulations, and specific actions. Finally, we reflect on the future of the movement given widespread concern about global warming and destruction of cultural and biological diversity.