Cities need forests. The network of woodlands, groups of trees and individual trees in a city and on its fringes performs a huge range of functions, such as: regulating climate; storing carbon; removing air pollutants; reducing the risk of flooding; assisting in food, energy and water security; and improving the physical and mental health of citizens. Forests enhance the look of cities and play important roles in social cohesion; they may even reduce crime. This edition of Unasylva takes a close look at urban and peri-urban forestry - its benefits, pitfalls, governance and challenges.