Foliar fertilizer sprays are increasingly used in agriculture for improving crop yield and quality, and also for improving plant performance under potential abiotic and/or biotic stress threats. This chapter focuses on new findings related to the nature of plant surfaces as a barrier for the absorption of agrochemicals. The chapter describes current knowledge on mechanisms of absorption by plant organs as affected by the structure and chemical composition of the surfaces. The chapter discusses key aspects related to the properties of foliar spray formulations for improving and standardizing the performance of the treatments under variable environmental conditions, with examples of recent foliar fertilization studies in relation to plant ontogeny. The chapter looks ahead to future trends for the application and development of foliar sprays as tool for sustainable agricultural production, as well as providing suggestions on where to look for further information.