A Green Wall is a wall, either free-standing or part of a building, that is partially or completely covered with vegetation and, in some cases, soil or an inorganic growing medium. The concept of the green wall dates back to 600 BC with the Hanging Gardens of Babylon. More recently, the larger green walls concept has been utilized with innovative hydroponics technology. The vegetation for a green façade is always attached on outside walls; with living walls this is also usually the case, although some living walls can also be green walls for interior use.For living walls there are many methods including attaching to the air return of the building to help with air filtration. They are also referred to as living walls, biowalls, vertical gardens, green walls and so on.