Water potential is the principal driving force for the movement of water through soils and plants, and directly influences plant physiological responses. The relationships between water potential and water content in plants and soil have long been of interest, and there is increasing focus on understanding how these fundamental measures of water are linked at larger spatial and temporal scales. In this Perspective, we explore how the theory of pressure-volume relationships can be applied at ecosystem scale. We define and evaluate the concept and limitations of the ecosystem pressure-volume curve (EPV), and discuss practical ways to construct EPVs with existing data. EPVs were generated from equilibrium water potentials and water content of the above ground biomass of nine plots including tropical rainforest, savanna, temperate forest, and a long-term Amazonian rainforest drought experiment. Initial findings suggest high levels of consistency among sites where the steady-state ratio of water:biomass appears to be approximately 1:3, while ecosystem values of relative hydraulic capacitance and accessible water storage do not vary systematically with biomass. The EPV reveals useful trends across ecosystems, providing a thermodynamically consistent steady-state view of ecosystem form and function, and a biophysically robust basis for the interpretation of microwave remote sensing data.