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This thesis investigates the thermal effect of green roofs and green walls on the built environment and investigates whether they could be used in existing cities so as mitigate the heat island effect. In order to estimate this for various climates, a prognostic, micro-scale, two-dimensional heat and mass transfer model has been developed. In the first chapter the aim, objectives and methodology of the research are established. In the second chapter a literature review of the causes and the consequences of the heat island effect is made, as well as a critique on the use of urban parks for mitigating raised urban temperatures. In the third chapter, a research is carried out where, when and why roofs and walls were covered with vegetation. In the fourth chapter the development of the one-dimensional model is presented, for heat and mass transfer in building materials, a soils, plants and the air. The model is validated with an experiment conducted at a concrete and a vegetated test cell. A comparison is made between green roofs and conventional concrete ones, as well as with white coated roofs for different climates. In the fifth chapter the one-dimensional model is developed into a two-dimensional one and the microclimatic heat and mass transfer model of a typical urban canyon is established. In the sixth chapter, the model is used to investigate the effect of green roofs and green walls for various climates, geometries, canyon orientations and wind directions. From this parametric analysis, an investigation is done on how the raised urban temperatures could lower when the building envelope is covered with vegetation, which could lead to energy savings for cooling and more comfortable outdoor conditions. In the last chapter, conclusions are drawn from the results of this thesis and further research is proposed.