A growing tendency in the 21st century is eco-architecture, also termed sustainable architecture; buildings with features which conserve heat and energy, and sometimes produce their own energy through solar cells and windmills, and use solar heat to generate solar hot water. They also may be built with their own wastewater treatment and sometimes rainwater harvesting Some buildings integrate gardens green walls and green roofs into their structures. Other features of eco-architecture include the use of wood and recycled materials. There are several green building certification programs, the best-known of which is the Leadership in Energy and Environmental Design, or LEED rating, which measure the environmental impact of buildings.