Green roofs have become increasingly accepted as a tool for contributing to the sustainability of cities throughout the world. Green roofs can help to counteract the effect of impervious surfaces in urban areas, where sidewalks, roads, rooftops, parking lots, and runways have historically been constructed using materials such as asphalt, concrete and stone, which repel water and prevent precipitation from seeping into soil. Because approximately 40 percent of the impervious surface area in our cities is located on rooftops, vegetated rooftops can have an important beneficial impact on the urban environment by reducing and controlling storm water runoff. Green roofs and living walls can act as filters for both air and water. They can also play an important role in reducing energy consumption for both heating and cooling, and in reducing the effects of urban heat islands. In addition, green roofs and living walls can contribute public green spaces to cities.