Plant growth-promoting rhizobacteria (PGPR) are defined as beneficial root-colonizing bacteria. Hence, the PGPR are a small subset of native rhizosphere bacteria. Root colonization is an active process by which bacteria survive inoculation onto seeds or plant parts and then multiply on seeds and roots in response to exudates from the plant. The underlying concept is that by introducing specific highly effective strains of bacteria, the balance of root-associated bacteria change with two types of possible beneficial results to the plant. PGPR which promote plant growth are commonly called biofertilizers. Those which provide biological control are commonly called biofungicides and require registration by the EPA for sale in the U.S. When considering the use of introduced bacteria such as PGPR, one must be aware of the strain concept, which means that each isolate of bacteria can impact plant differently. Hence, although there are several PGPR products containing different strains of one species, such as Bacillus subtilis, we cannot consider all strains of this species to be beneficial. Therefore, continual development of new PGPR-based products is based on finding new strains with different mechanisms and activities. Much of the research with PGPR was originally done with fluorescent pseudomonads, but these bacteria have not successfully been formulated for delivery to agriculture. In contrast, bacilli form endospores that allow development of formulations with a multi-year shelf life, and hence, most commercially available PGPR products consist of strains of bacilli. Mechanisms of growth promotion by PGPR-biofertilizers include production of plant growth activators such as IAA, release of volatile growth-stimulating compounds, and inhibition of deleterious rhizobacteria via competition for iron. Mechanisms of biological control by PGPR-biofungicides include production of antifungal compounds, including many types of antibiotics, and induction of host defenses (induced systemic resistance).