Trees in the urban environment have been proposed as one mechanism to reduce air pollution and the impacts of the UHI in cities. There are now policies and initiatives in place globally to increase the canopy cover in our neighbourhoods. Trees have numerous other benefits or ecosystem services, for example providing habitats, visual amenity and storm water management and form an integral part of the wider network of green infrastructure in our towns and cities. This chapter summarises the mechanisms by which trees have the potential to provide two regulating ecosystem services: improved air quality and reduced temperature. It also explores some of characteristics of trees and their planting location that are associated with the greatest benefits.