Bacteria and the viruses that infect them, known as bacteriophages (phages), are in a constant evolutionary arms race in which both microorganisms adapt and evolve defence and evasion mechanisms against each other. Until a few years ago, bacterial immunity against phages was considered to comprise mainly of CRISPR–Cas and restriction–modification systems, but recent studies have shown that bacteria encode more than a hundred kinds of defence systems. How these defence systems protect against phages and how phages may have evolved to evade host immunity remain unknown.