This book containing 22 chapters is divided into 3 sections. The chapters in Section I present overviews of the phage world that are designed to provide updates of relevant information for those with more than a cursory knowledge of phage lore and sufficient background for the non-cognoscenti to assist in understanding the chapters focusing on specific pathogens and their phages. The chapters in Section II explore the role of phages in the biology and pathogenicity of specific infectious bacteria. It is these chapters that obviously provide the rationale for initiating this endeavour. The chapters in Section III present new information regarding applications of phage biology in biotechnology as well as a critical assessment of therapeutic and diagnostic use of phage.