A green building is an enclosed structure that meets a recognized standard for high energy efficiency or environmental sustainability. Normally a building must receive a rating from a respected organization dedicated to certifying green buildings, such as the USGBC (LEED ratings) or the Building Research Establishment (BREEAM ratings). Buildings may achieve this status by meeting various quota and by possessing a certain number of green features.

One of the highest goals of green building certification is to encourage carbon neutrality, which is achieved when net carbon emissions resulting from the building's presence are zero or less. This can be achieved in various ways, including on-site energy production through sustainable technologies such as photovoltaic panels or wind turbines, often in combination with energy-saving features such as lighting control, solar water heating, or efficient insulation.

Green building certification also looks at the environmental effects that are tangentially related to a building's use. For example, buildings accessible through mass transit may be rated higher than buildings where most occupants must arrive by car.

New buildings with green ratings are often very modern in appearance because of the levels of technology required; however there are no inherent design constraints that force a particular style. Many older buildings earn very high green ratings after an environmental upgrade; in fact, re-use of existing buildings is considered a far more sustainable practice than new construction because of the energy, land, and natural materials lost in new construction.