The current ecological condition of cities in respect to the surrounding countryside (higher temperatures and CO, levels, drought, photochemical pressure and particulate matter pollutants), may anticipate the effects of climate change at a broader scale. However, the condition of urban trees can be exacerbated by some specific factors within cities, such as mismanagement and the direct impact of human activities. The acclimation and adaptation strategies shown by urban tree species may be similar to those already known for climate change, including the substitution of the current tree species with better adapted genotypes (native or not native) and the adoption of techniques aimed at improving stress resistance. In this paper, we review the main ecological factors affecting the physiology of urban trees, with a special reference to Mediterranean conditions. We summarise the criteria to select the most suitable trees, such as phenotyping and the identification of functional traits associated with stress resistance, physiological functionality and ecosystem services. The role of tree diversity in maximising ecosystem stability and services is introduced.