This chapter focuses on viruses that infect bacteria-so-called phages. We first introduce the two primary life cycles of phages, which classify them as lytic or temperate phages. Phages are both of importance as a model system in the lab and as ubiquitous agents in nature. We discuss the famous Luria-Delbrück experiment, in which bacteria and phages were used to provide fundamental insights into the random nature of mutations, and briefly explain the importance of viral-bacterial interactions in the ecosystem by the example of phages in the ocean. We then explore key components of the bacterial defence system against phages. One of them is the CRISPR-Cas system, a kind of adaptive immune system, which researchers have developed into a revolutionary tool for genome editing.