There has been an increasing demand for food throughout the world. If global food production is to keep pace with an increasingly urbanized growing population while formulating new food production strategies for developing countries, the great challenges to modern societies is to boost plant productivity in an environmentally sustainable manner.Microbes in this context provide the best alternative. Also they can be useful in reducing pollution of chemical fertilizers. Microorganisms as biofertilizers and biocontrol agents have proved to be useful in increasing crop productivity and to control diseases in an environment friendly manner. This book deals with microorganisms, their beneficial effects on plants and mechanism of their action on plant growth and productivity in a simplified way.