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This dissertation was written as part of the MSc in Sustainable Agriculture and Business at the International Hellenic University. The continuously increasing demand for human food, consequently creates the need for increase of edible plant and livestock production. The demand for the replacement of chemical fertilizers and the introduction of alternatives in order to enhance crop productivity led to the research and use of microorganisms that affect positively the soil, the rhizosphere and finally the plants promoting its development. Such microorganisms are called microbial inoculants and they are mainly a) plant growth promoting rhizobacteria, b) arbuscular mycorrhizal fungi and c) endophytes. The inoculation of these inoculants of plants with main concern for livestock as maize (Zea mays), wheat (Triticum aestivum), barley (Hordeum vulgare), oats (Avena sativa), soybean (Glycine max), sorghum (Sorghum spp.), alfalfa (Medicago sativa) and trefoil (Trifolium spp.) revealed their beneficial effect on most cases improvement of the quantity and quality of plant food, although in some cases they had a negative or neutral impact. It is shown that these microorganisms can promote plant growth in terms of root and aerial parts parameters, yield production as well as nutrient uptake from the soil and so to enhance the quality of the feed. Moreover, they managed to alleviate stressful environmental conditions as drought, salinity and toxicity of soils polluted by heavy metals. Especially, our study showed that bacteria of the genera Bacillus and Pseudomonas revealed their beneficial effects on maize, wheat, barley and soybean in nutrient deficiency, environmental stressed and non-stressed conditions. Additionally, fungal genera Glomus promoted plant growth and yield production of maize, wheat and soybean under every investigated soil condition. Furthermore, bacteria of genera Arthrobacter, Rhodococcus, Burkholderia and Aeromonas reduced the nutrient deficiency, especially for N, P and K. Environmental stresses caused by drought were reduced or neutralized by inoculants of genera Pseudomonas, Glomus, Azospirillum, Enterobacter, Burkholderia and Acaulospora and those caused by salinity by Hartmanibacter, Glomus, Bacillus and Halobacillus respectively. Bacteria as Bradyrhizobium diazoefficiens and Enterobacter ludwigii and fungi as Claroideoglomus etunicatum alleviated the pollution of soil by heavy metals and improved plant development. In addition, it was revealed that most microorganisms do not have a plant specificity as they inoculated and affected more than one of the plants concerning us. Specifically, Pseudomonas fluorescens, bacteria of genera Burkholderia, Arhtrobacter and Rhodococcus promoted maize’s and wheat’s development and productivity. Glomus spp., Azospirillum spp. and Enterobacter spp. improved maize’s, wheat’s and soybean’s performance. Finally, Paenibacillus spp. and Pantoea spp. affected positively the growth and performance of maize, wheat and barley and especially Paenibacillus polymixa revealed its ability to induce systemic resistance against pathogenic microorganisms in wheat. In summary, the application of these microbial inoculants in maize, wheat, barley and soybean is a promising strategy for the enhancement of productivity of these plants and the livestock fed with them.