The world is urbanising at an unprecedented rate – over half the world’s population live in cities. This proportion is even higher in Europe, where nearly three quarters of people live in cities. Globally, the urbanised population is projected to increase to 66% by 2050, with 95% of urban expansion expected to occur in the developing world. Urban trees provide a range of ecosystem services, thus they form a vital component of liveable cities. But urban forests are threatened by land use conflicts, climate change, intensive human use and abuse, as well as pests and disease – and as a consequence, the social, economic and environmental benefits relied upon by citizens are threatened. Fortunately, species diversity, diversity within species, age and structural diversity can support urban forest health, and thus ensure optimal and long-term provision of ecosystem services in the face of such threats.