Grafting, a surgical technique to attach genetically different shoots and roots (scions and rootstocks) allows “designer root systems” to enhance agricultural productivity and sustainability. Rootstocks improve plant nutrient relations by increasing nutrient capture and/or nutrient use efficiency, by multiple mechanisms including altered root morphology. Moreover, rootstocks can enhance water uptake and/or diminish water loss according to changes in root form and function, and root-to-shoot phytohormonal signalling. While the role of root-to-shoot ABA signalling in effecting stomatal closure is equivocal, root-sourced cytokinins and ACC regulate shoot senescence and vegetative growth respectively. Rootstock-mediated crop improvement offers opportunities to enhance crop resource use efficiency, especially in the developing world.