New standard procedures for assessing the efficiency of water use in irrigation systems have been compiled by the International Commission on Irrigation and Drainage (ICID). They are based on the efficiencies of the three separate operations by which water is moved through an irrigation system: conveyance, distribution, and field application. Definitions of these efficiencies are given. Also defined are the tertiary unit efficiency and the overall or project efficiency, which are combinations of the above separate efficiencies. The terminology used in the definitions to describe irrigation units, water supply canals or conduits, and related structures is explained.