Half the world's children live in cities and the proportion is growing. Their environment critically determines their futures and the world they will make as adults. This text, by an interdisciplinary team of international child-environment authorities, explores how crucial the relationship of the young and their surroundings is. Covering eight countries, it shows the enormous benefits - for them, for the wider society and for the future - of involving children, especially from underprivileged communities, in planning and implementing urban improvements. It continues and updates Kevin Leech's pioneering 1970s MIT project, Growing Up in Cities.