In the context of the “New Green Revolution,” innovative agricultural practices have to be identified to sustainably improve the traditional cultural approaches already used in the green revolution and to apply efficient technologies to solve actual agricultural problems (more particularly in the developing countries) but without the use of chemical pesticides and fertilizers. To reach this objective, soil microbes represent a largely unexplored resource to promote agricultural yield and productivity in the context of sustainable farming practices. Among these beneficial microorganisms, arbuscular mycorrhizal fungi (AMF) form symbiotic association with ~80% of vascular plant species. These efficient symbionts are considered as a key group of soil microorganisms able to improve P uptake by plants. In addition, AMF have the potential to improve plant defense against plant pathogens and to promote plant tolerance against abiotic stresses. AMF are primary biotic soil components which, when missing or impoverished, e.g., due to anthropic input, can lead to a less efficient ecosystem functioning. Many environmental factors can affect the impact of AMF inoculation on the plant growth including the fungal species compatibility with soil characteristics and the interactions between the fungal inoculant and other soil microorganisms. Some microorganisms, named mycorrhiza helper bacteria (MHB), can facilitate the establishment and the functioning of the AMF symbiosis by stimulating spore germination, mycelial growth, root colonization, or sporulation but also by reducing stresses that could impact AMF symbiosis. The aim of this chapter is to review mechanisms implemented by MHB to promote the AMF establishment and to enhance the efficiency of the mycorrhizal effect on the plant growth. Using Mycorrhiza Helper Microorganisms (MHM) to Improve the Mycorrhizal Efficiency on Plant Growth.