Energy and water are critical resources that are inextricably and reciprocally linked. The production of energy requires large volumes of water, and the treatment and distribution of water depends upon readily available, low-cost energy. For example, electricity production from thermoelectric power plants can use ∼140,000 million gallons of water per day for cooling—accounting for 39% of all freshwater withdrawals in the nation, second only to agriculture in the United States (Figure 1). Significant amounts of water are also needed for hydropower, extraction/refining of minerals for energy, and bio-fuel production. Electrical energy, on the other hand, is needed for water treatment (e.g., desalination, wastewater), pumping, and distribution. The amount of electricity used in water and wastewater industries is equivalent to the amount used in chemical, petroleum refining, and paper industries. These interdependencies, coupled with increasing demands for energy and diminishing availability of freshwater supplies, pose significant challenges to ensure the sustainability of these two critical resources. Examples of the interrelationships between energy and water use are shown in Figure 2.