Measuring crop water requirements is an essential part of managing agricultural water in crop fields. In this chapter, we propose a research approach for the determination of crop water requirements that integrates soil, plant and atmospheric measurements. Selected methods for estimating crop water requirements (lysimetry, atmospheric, plant- and soil-based methods), their spatial scale and time frequency, and their applicability are discussed. A case study is presented for an apple orchard in South Africa, where atmospheric (eddy covariance system), plant (sap flow) and soil water content measurements (time domain reflectometry) were used to determine seasonal evapotranspiration (ET) and crop water requirements, transpiration and the extent of over-irrigation (deep percolation). Splitting ET into crop transpiration and soil evaporation allowed us to determine basal crop coefficients Kcb, which are more transferable than the Kc crop coefficients and less dependent on management practices. The chapter also provides recommendations for further research and additional literature.