The growing interest in PGPRs emerges from the wish to shed more light on a complex and fascinating biological system and from the potential these organisms have to increase productivity of crop cultures. The strains that exhibit a specific desired characteristic are selected, tested, and then used as an element in agricultural practices, though so far these strategies have not been extensively adopted ? in spite of the several well-succeeded examples mentioned in this chapter. In a scenario where a greater number and more efficient bacterial strains are used, agricultural applications of PGPRs shall increase. Efforts made in the last decade have provided better insight into PGPRs and the mechanisms these bacteria utilize to promote plant growth, increasing the commercial interest in these strains (Glick 1995). While PGRPs have been characterized within several different bacterial taxa, many PGPRs developed for commercial applications are Bacillus species. These products utilize bacilli in their endospore form, which lends population stability during formulation and shelf life. Among bacilli, the strains of B. subtilis are the most widely used PGPRs, due to their capacity to produce antibiotics and consequent reduction in disease occurrence, when they are used as components of seed treatment strategies (Kokalis-Burelle et al. 2006).