Soil microorganisms, such as bacteria and fungi, produce an array of compounds including volatile compounds. These are compounds that can easily evaporate in the soil, facilitating interactions between soil microorganisms without physical contact. Hence, volatiles play important roles in plant-plant, plant-insect, plant-microbe and microbe-microbe interactions. For this PhD thesis I studied the composition of volatiles produced by beneficial and pathogenic soil microorganisms and the effects of these compounds on growth, development and resistance of plants. Microbial volatiles stimulated growth of plants, including different crop plants, both under lab conditions and in soil. Molecular analysis of plants exposed to the microbial volatiles further revealed possible mechanisms underlying plant-microbe interactions. Microbial volatiles provide an exciting new strategy for biostimulation and biocontrol. The understanding of their ecological functions will contribute to sustainable agriculture by revealing novel mechanisms for improving crop production.