Cnidarians, mostly soft-bodied water organisms, produce several classes of toxins deployed in biological warfare or signalling. Cytolytic toxins, that form pores in cell membranes, form a significant part of their "weaponry". Here, we describe the physiological relevance of membrane permeabilization, and present basic data on those proteinaceous cnidarian cytolysins proven or presumed to damage cell membranes by pore formation. We describe cytolysins that have been at least partially characterized, both functionally and structurally.