"Sustainable engineering" and "green engineering" are terms that are often used interchangeably. The main difference between the two being that green engineering is "optimized to minimize negative impacts without exhausting resources available in the natural environment" and sustainable engineering is "more directed toward building a better future for the next generations". The idea of sustainable development became intertwined with engineering and chemistry early in the 21st century. One often cited book that brought the idea of sustainable development to engineers was the publishing of: "Sustainable Infrastructure: Principles into Practice", written by Charles Ainger and Richard Fenner.