Endophytic microorganisms that can grow in the intercellular spaces of higher plants are recognized as one of the most chemically promising groups of microorganisms in plant protection.Endophytes offer a wide range of benefits to plants such as growth promotion reducing disease severity inducing plant defense mechanisms producing anti-herbivore products biologically fixing nitrogen, and increasing plant mineral uptake.This book describes the effective use of Bacillus subtilis as a valuable candidate in the context of biological control.