Large numbers of bacterial and fungal endophytes have been reported from different plant tissues: roots, nodules, leaves, flowers and sprouts of legumes, with numbers ranging from few to more than 150. Endophytes can accelerate seedling emergence, promote plant establishment under adverse conditions and enhance plant growth. Endophytic microbes promote plant growth by helping plants in acquiring nutrients, e.g. via nitrogen fixation, phosphate solubil- ization or iron chelation, by preventing pathogen infections via antifungal or antibacterial agents, by outcompeting pathogens for nutrients by siderophore production, or by establishing the plant’s systemic resistance. Further growth promotion is affected by producing phytohor- mones such as auxin or cytokinin, or by producing the enzyme 1-aminocyclopropane- 1-carbo- xylate (ACC) deaminase, which lowers plant ethylene levels. For establishment of endophytes in different tissues, endophytic microbes must be compatible with the host plants and able to colonize the tissues of the host plants without being recognized as pathogens. A particular bac- terium or fungus may affect plant growth and development using one or more of these mecha- nisms, and they may use different mechanisms at various times. The population density of endophytes is highly variable, depending mainly on the microbial species and host genotypes, developmental stage and environmental conditions. Genotypic and cultivar specific endophytes have also been reported. The quantum benefit derived by plants from an endophyte and vice versa is still not clear. It seems that the endophytic genus or species best adapted for living in- side a plant is naturally selected. Here, we concentrate on soil or rhizosphere-derived endophy- tes recruited out of a large pool of soil or rhizospheric microbes. Some endophytes are more aggressive colonizers and displace others, but seeming lack of strict specificity has been observed.