Streptolysin S (SLS) is a potent cytolytic toxin and virulence factor that is produced by nearly all Streptococcus pyogenes strains. Despite a 100-year history of research on this toxin, it has only recently been established that SLS is just one of an extended family of post-translationally modified virulence factors (the SLS-like peptides) that are produced by some streptococci and other Gram-positive pathogens, such as Listeria monocytogenes and Clostridium botulinum. In this Review, we describe the identification, genetics, biochemistry and various functions of SLS. We also discuss the shared features of the virulence-associated SLS-like peptides, as well as their place within the rapidly expanding family of thiazole/oxazole-modified microcins (TOMMs).