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ET-based irrigation scheduling has grown considerably in recent decades as the technology to collect data and disseminate information has improved. A huge advance occurred with the publication of the ASCE-EWRI standardized reference ET (ETref) equations, which provided an equation to account for weather effects on ET by quantifying evaporative demand. Well-watered crop evapotranspiration (ETc) is estimated as the product of ETref and an appropriate crop coefficient (Kc) value that accounts for the difference in ET between the reference and crop surfaces. The estimation of ETc is an important goal for irrigation researchers, but advances are difficult to make because of climate and irrigation management effects on Kc values. For efficient irrigation management, there is also a need to account for water and salinity stress effects on the actual crop evapotranspiration (ETa) when computing irrigation schedules. Many plant physiologists are investigating the use of plant-based measurements for regulated deficit irrigation of orchard and vine crops. ET information, however, is still necessary to properly interpret the plant-based measurements and to modify the irrigation management. In urban irrigation, water is applied rapidly and runoff is often the limiting factor determining irrigation timing and amount. Drip irrigated and shallow rooted crops are also frequently irrigated, so near-real time and forecast ETc estimates are needed to optimize irrigation management. In this paper, information on Kc research in California, ETo forecasts, and in-situ ET measurements is presented.