Publisher Summary Several different families of genetically, structurally, and functionally related bacterial pore-forming cytolysins have been identified. The RTX toxins, which are found in a variety of Gram-negative pathogens, represent one of the largest of these toxin families. The designation "RTX toxins" thereby refers to the presence of a characteristic and functionally important tandem array of glycine-rich nonapeptide repeats in these toxins. RTX toxins are synthesized as inactive protoxins that require posttranslational activation (modification) prior to export from the toxin-producing bacteria. This activation is directed by a specific accessory protein that is co-synthesized with the protoxin. So far, two RTX toxins, E. coli a-hemolysin and B. pertussis AC toxin, have been shown to be activated by covalent fatty acylation of specific lysine residues. This chapter reviews the structure and mode of action of RTX toxins. A large quantity of new data regarding the structure and mode of action of RTX toxins has been compiled in the past years. Several things are, however, still poorly or incompletely understood. Examples are the mechanism of pore formation, the function of the fatty acylation and of the binding of calcium ions to RTX toxins, and the processes involved in the RTX toxin-mediated activation of host cells. Current work is aimed at further clarifying these issues.