The sustainability of water systems for urban areas can be addressed only when the full human hydrologic cycle is defined and, ultimately, integrated into a single process. By analogy to natural hydrologic cycles, the human hydrologic cycle tracks water from its source in natural water bodies, through every engineered component of transportation, treatment, distribution, use, collection and treatment of wastewater, back to its ultimate discharge into natural water bodies. Environmental, economic and social sustainability criteria are defined and include, for example, per capita water supply, discharge water quality, energy use per volume, and expenditure per capita per volume. The key factor driving the sustainability of urban water systems is per capita demand. Demand can be reduced in part by identifying different water qualities required for different uses. Both advanced wastewater treatment processes, and a limited amount of rainwater harvesting, can provide high quality water for non-potable use, decreasing the need for water from natural water bodies. When demand is decreased, the total energy required to drive this cycle decreases. Additional energy, enough to operate the wastewater treatment process, can also be obtained by converting the organic constituents in wastewater into hydrogen and methane. Changing urban morphology may facilitate the greatest reductions in demand and energy use and might present one of the best opportunities to achieve sustainability.