This chapter presents computation and modelling approaches of crop water use and irrigation water requirements and discusses concepts of water use performance and productivity. The chapter also describes the main features of soil water balance and irrigation scheduling models. It focuses on the SIMDualKc model which accurately partitions evapotranspiration into actual crop transpiration and soil evaporation. The chapter includes examples of the application of modelling in horticulture, field crops, olive orchards and vineyards as well as intercropped wheat and sunflower. It also includes an example of modelling when saline water is used, thus with identification of impacts on actual transpiration.