Chicago’s most famous rooftop garden sits atop City Hall, an 11-story office building in the Loop. City Hall and the adjacent Cook County building appear to most people as one building spanning a city block bounded by LaSalle, Randolph, Clark and Washington streets. First planted in 2000, the City Hall rooftop garden was conceived as a demonstration project - part of the City's Urban Heat Island Initiative - to test the benefits of green roofs and how they affect temperature and air quality. The garden consists of 20,000 plants of more than 150 species, including shrubs, vines and two trees. The plants were selected for their ability to thrive in the conditions on the roof, which is exposed to the sun and can be windy and arid. Most are prairie plants native to the Chicago region.

Like all green roofs, the City Hall rooftop garden improves air quality, conserves energy, reduces stormwater runoff and helps lessen the urban heat island effect. The garden's plants reflect heat, provide shade and help cool the surrounding air through evapotranspiration, which occurs when plants secrete or "transpire" water through pores in their leaves. The water draws heat as it evaporates, cooling the air in the process. Plants also filter the air, which improves air quality by using excess carbon dioxide to produce oxygen.

The rooftop garden mitigates the urban heat island effect by replacing what was a ballasted, black tar roof with green plants. The garden absorbs less heat from the sun than the tar roof, keeping City Hall cooler in summer and requiring less energy for air conditioning. The garden also absorbs and uses rain water. It can retain 75% of a 1 inch rainfall before there is stormwater runoff into the sewers.