Several experiments on individual buildings in California and Florida show that reflective (cool) roofs reduce air-conditioning energy use between 10 percent and 50 percent. The savings, of course, are strong functions of the thermal integrity of building and climate conditions. Darker roofs more quickly warm the air over urban areas, leading to the creation of summer urban ''heat islands.'' On the community scale, increasing the albedo (solar reflectivity) of roofs can limit or reverse an urban heat island effectively and inexpensively. This publication discusses the literature data and new research efforts in analyzing the impact of cool roofs on buildings' cooling and heating energy use.