Cities and towns are often affected by the urban heat island effect, whereby air temperatures are higher than those in surrounding rural environments. This Research Note describes the negative impact that elevated urban temperatures can have on human thermal comfort and health and how urban green infrastructure can help lessen this impact. Drawing on recent research, two particular aspects of green infrastructure are explored. Firstly, the cooling effectiveness of urban greenspaces is examined. Secondly, the role urban trees play in providing cooling and the factors that may influence this benefit are highlighted. This Note gives examples of how the urban environment can limit cooling from vegetation, and provides guidance as to how these limitations can be reduced. Current scientific knowledge of strategies to maximise cooling and the extent to which this knowledge is being translated into practice are discussed as are the measures which have been adopted to help value this benefit. In light of climate change, the need for cooling by trees and greenspaces is expected to increase even in temperate climates such as that of the UK. Green infrastructure planning and development should embrace greenspace design and tree placement that facilitate such cooling, as well as include tree species with high cooling ability and ensure they are provided with enough space and resource to grow and function. Further research on the design strategies that lead to maximum cooling is required. Communication between researchers, practitioners and policymakers should be strengthened.