Water management faces great challenges over the coming decades. Pressures include stricter water-quality standards, increasing demand for water and the need to adapt to climate change, while reducing emissions of greenhouse gases. The processes of abstraction, conveyance and treatment of fresh water and wastewater all demand energy. Energy use in the water sector is growing, yet its importance is under-recognized, and gaps remain in our knowledge. Here we define the need to integrate energy use further into water resource management and identify opportunities for the water sector to understand and describe more effectively its role in greenhouse-gas emissions.