Bacteriophages (phages) are the viruses of bacteria and biofilms that represent a frequent niche for bacteria, where they are embedded in extensive extracellular polymeric substances (EPS) and can be structured into complex microcolonies. As a consequence of the resulting spatial structure and heterogeneity, phage-bacterial interactions within biofilms can be more complicated than those between phages and planktonic bacteria. This book presents and discusses research which provides a better understanding of the biology of phages interacting with biofilms.