Cities typically exhibit higher temperatures than do rural areas, due to the absorption of solar energy by asphalted streets and concrete buildings, as well as the diminished release of latent energy by evapotranspiration. Among ways of mitigating the so-called 'urban heat island' is the enhancement of vegetation along streets and in parks, and the use of soil-simulating substrates on flat roofs to grow appropriate vegetation. The latter method, often called 'green roofs,' offers the potential for reducing the requirements for storm-water drainage as well as for thermally insulating such buildings. A review and critique of these approaches will be presented, along with modeling studies and actual examples.