Plant water use is inextricably linked to both agricultural productivity and sustainable water resource management. Unlike plants in natural ecosystems, crops and horticultural species benefit from the active management of their growing environments. Tremendous advancements have been made towards more efficient irrigation systems and higher yielding cultivars; however, technological solutions are limited by the physical and biological processes governing plant water use. This chapter discusses the fundamentals of water movement through the soil-plant-atmosphere continuum and the mechanisms by which vegetation can control water loss to the atmosphere, i.e. evapotranspiration (ET). It also examines what drives ET, and the equations used to model and predict it. The chapter then reviews standard and advanced techniques for measuring plant water use across a range of scales, from leaf to landscape. Finally, it describes the emerging green water/blue water paradigm and how water embedded in agricultural products makes water management for sustainable agricultural a truly global issue.