In crop plants grain yield is dependent on biomass which in-turn determined by the total water use and its efficiency (WUE). Therefore in a crop improvement programme, it is always useful to identify genotypes where WUE is high despite relatively high transpiration. Such genotypes will be more productive by virtue of better biomass accumulation. This monogram highlights the importance of determining genetic variability in WUE and transpiration simultaneously in several crop species through oxygen (�O) and carbon (�C) isotope signatures. This approach is highly useful in quantifying transpiration and mesophyll factors associated with WUE which is a key factor in assessing economic yield both under irrigated and rainfed conditions. Stable isotope signatures act as indicators of growth & yield (both acquired and adaptive) traits and provide strong impetus in screening efficient donor lines (phenotyping for WUE & yield) under any given water inputs, hence could form a basis for strategic breeding in crop plants. Dual isotope (�C/�O) technique paved the way for identifying water use efficient crop genotypes with improved biomass/yield