This talk will present a structural biology view of the MACPF/CDC proteins, a superfamily in the evolutionary arms race between hosts and pathogens. Natively folded proteins usually have a single fold, with minor variations related to their function, but pore-forming proteins break the rules by undergoing a complete change in character from water-soluble monomers to large, membrane-inserted oligomers. What have we learned about this intriguing family of misbehaving proteins? With the advances in cryo-electron microscopy in the last decade or so, the 3D structures of both soluble and pore forms have been determined at increasing levels of resolution. The structures reveal the moving parts and key residues involved in the transition, but the underlying mechanisms and energetics of the transition are still not fully understood. Why is this knowledge useful? In pathology, pore forming proteins play important roles in the immune response, and adjusting the level of this response can be critical in medical treatment. In agriculture, crops can be defended against major pests by engineering them to express highly targeted pore forming proteins specific to the attacking pathogen. The lecture will focus on what we can learn from the structures of soluble, pre-pore and pore forms.