Clean water is becoming an increasingly scarce commodity. Green buildings aim to develop systems to minimise the consumption and pollution of this resource. Careful design is used to develop rainwater harvesting, plumbing and ecological sanitation systems that enable buildings to be self reliant for their water needs and avoid polluting water. This reduces the requirement for large-scale water and sanitation infrastructure that consumes energy and can be highly wasteful.This chapter describes water systems used in green buildings and sets out some objectives that could be aimed for. It also outlines some calculations that can be used to design water systems in green buildings. Finally, aspects of green building water systems are described, so that designers can select, and work with, the most appropriate of these to develop high performance sustainable water systems in buildings.