Bacteriophages or phages are the viruses that infect bacterial cells. They replicate within these bacteria. Bacteriophages can be categorized on the basis of their morphological characteristics, nucleic acid content, site where they are usually found, and the bacterial target. Phage therapy is a specialized application of bacteriophages used for treatments wherein bacterial infections are treated using bacteriophages. The genomes of bacteriophage are used to encode highly pathogenic bacterial toxins including cholera toxin in Vibrio cholerae and diphtheria toxin in Corynebacterium diphtheria. Despite its benefits, there are some risks associated with phage therapy. For instance, it may lead to development of antibiotic resistance and reduced activity due to immune system response. This book contains some path-breaking studies on bacteriophage biology. The various advancements in bacteriophage research have been glanced at and their applications as well as ramifications have been analyzed in detail. The book will serve as a valuable source of reference for the graduate and postgraduate students.