Bacteria are the most abundant, versatile, and oldest organisms on earth. They play vital roles in the world's ecology and inhuman industry and medicine. In this book, we present an introduction to some of the basic principles of bacteriology. We describe molecular genetics regarding bacteria and how to interact between plants and bacteria. Some bacteria, like plant growth-promoting rhizobacteria (PGPR), play an important role in sustainable and safe agriculture. The various classes of antibiotics, their mechanisms of action, and the nature of bacterial resistance are discussed. We designate the various mechanisms of gene transfer in bacteria including the manipulation of DNA in vitro. We describe phytopathogenic bacteria and phytoplasmas diseases and their importance. We define some bacteria that play a significant role in crop protection from environmental stress (PGPR), biofertilizers, biopesticide and improve crop yield.