Water Use Efficiency (WUE) expresses the trade-off between carbon assimilation (or subsequent carbon storage) and water release, two concurrent gas fluxes essential for plant functioning. Here, we review metrics that have been introduced to quantify WUE across spatial and temporal scales and corresponding observational methods. The physiological and environmental dependencies of WUE for crops and unmanaged ecosystems are discussed with a quantitative examination of observed and simulated WUE values. A constrained range of WUE across ecosystems and climates is highlighted. However, WUE increased considerably in the past century in response to growing atmospheric CO2 concentration and it will likely continue along the same trajectory in the near future.