This review presents a synthesis of the current state of knowledge of physiological processes, weather conditions and management factors affecting the responses of different crops to foliar fertilization with nitrogen and phosphorus. It is concluded that if foliar fertilization is carried out in the correct way under carefully optimized conditions, it will be possible to obtain higher nutrient efficiencies than is the case for conventional soil-based fertilizer applications. However, successful implementation of foliar fertilization requires careful optimization of the conditions for nutrient uptake across the leaf barriers as affected by the form of nutrient applied, the concentration of salts in the applied solution, the addition of adjuvant and the application time in relation to crop developmental stage and weather conditions. Foliar fertilization is more demanding with respect to technical knowledge and management skills than is the conventional use of solid fertilizers. If not carried out appropriately, foliar fertilization with nitrogen or phosphorus will imply a considerable risk of causing negative yield responses. An important target for future innovation will be development of new nutrient formulations, adjuvants and synergists, that can prolong the duration of the period in which the sprayed solution remains as a liquid on the leaf surface and ensure rapid and efficient nutrient uptake with minimal risks of scorch. In addition, it will be essential to develop new sensor and spraying technologies for precision agriculture, including drone-based systems that may enable frequent applications of relatively low nutrient doses under optimum weather conditions.