The need for new eco-friendly control methods against plant diseases and pests requires that the scientific research is focused on effective tools for a safe environment for humans and animal health. The systemic acquired resistance (SAR) represents a valid opportunity in plant natural protection. Particularly, biocontrol microorganisms (BCMs) can be used as inducers of SAR. At the same time, fungal BCMs are able to promote plant growth and development (so acting as plant growth-promoting microorganisms (PGPMs)) that in turn determine a higher tolerance against abiotic stresses, such as drought and salinity. The ability of BCMs and PGPMs of modulating plant defence mechanisms, like SAR, was demonstrated, but the details of this BCM-plant molecular crosstalk are poorly known and many defensive compounds are likely to exist but remain to be identified. For these reasons, BCMs and PGPMs can be defined as “biostimulant microorganisms”, able to foster plant growth and defence against pathogens throughout the crop life cycle, from seed germination to plant maturity. The aim of this chapter is to give an up-to-date overview on the recent breakthroughs in the use of biostimulant microorganisms on plants for improving crop vigour, yields and quality and for increasing plant tolerance against biotic and abiotic stresses.