Arbuscular mycorrhizal fungi are symbiotic to most terrestrial plants. The fungi are an important part of ecosystem affecting its efficiency. Among the most important effects of mycorrhizal fungi on the ecosystem efficiency is by alleviating different stresses such as pathogens, drought, salinity, compaction, etc. The fungi are able to enhance plant growth by increasing the uptake of water and nutrients and by producing plant hormones. Accordingly, in this book some of the latest finding and progress related to the effects of mycorrhizal fungi on the efficiency of ecosystem by alleviating soil stresses have been presented. Mohammad Miransari AbtinBerkeh Ltd. Company