Phages, viruses that infect bacteria and archaea, are the most abundant and genetically diverse biological entities in the biosphere, with a total population of approximately 10^31–33. In response to this pressure, prokaryotes have evolved a diverse battery of immune mechanisms to counteract phage attack. The most abundant defense systems, restriction-modification and CRISPR-Cas, have led to the development of revolutionary technologies for genetic engineering (recombinant DNA and CRISPR-Cas9 gene editing, respectively), and have been studied in great detail. More recently, over 100 new immune systems of bacteria and archaea have been discovered, many of which are the evolutionary precursors of human cell-autonomous immune pathways. The Symposium on the Immune System of Bacteria will cover the latest studies and findings of both old and new anti-phage defense strategies.