Part of a series of four entitled Urban Patterns for a Green Economy, this guide proposes that cities can act as agents for change that allow their large populations to live less wastefully. It considers how infrastructure systems can be viewed as an opportunity to shift cities onto a more sustainable path by paying close attention to the resources that pass through them, and the manner in which they support the activities of the city. Each city context differs, based on stage of development, pace of growth and available resources.