Pore-forming proteins are crucial armaments in the continuous battle between living organisms and the pathogens that threaten their fitness and survival. These proteins act on cells, which are the micrometre-scaled, basic units of all forms of life. Cells are separated and protected from their environment by a thin membrane. Pathogens such as bacteria can release pore-forming proteins ("toxins") that drill holes in the membranes of healthy cells in the host organism, to release nutrients for the bacteria, to invade these cells and/or kill them. Patients affected by bacterial pneumonia, for example, suffer from the devastating effects of such a toxin, pneumolysin, on lung tissue. The immune system, however, uses a similar mechanism to kill germs and infected or cancerous cells, thus preventing them from doing further damage to the organism. It secretes related, but somewhat different pore-forming proteins to perforate the membranes of such unwanted invaders. To perform these tasks, pore-forming proteins have developed sophisticated drilling mechanism. These proteins can convert from a soluble form in the aqueous, cellular environment into a very different form, in which 20-50 protein molecules assemble into a ring-shaped pore bound to the membrane. We can look at these forms with X-rays or electrons to deduce their three-dimensional structures. Thanks to such experiments, we now have a reasonably clear picture of the soluble proteins and their pore structure in the membrane. For some pore-forming proteins, scientists have even identified the changes inside the proteins which make this transition possible. However, if we wish to design drugs that prevent such pores from being formed, as in the example of bacterial pneumonia indicated above, it would be useful to know more about the steps in their formation. It is exactly this pore assembly that is still largely enigmatic. In this project, we will try to answer some specific questions about membrane pore formation. We would like to know how the proteins assemble on the membrane. Do they assemble one by one, or do they first form larger units that subsequently assemble in a pore? Do the proteins first need to assemble on the membrane, or can they dock in the membrane and assemble in pores afterwards? And at what point in this process will the membrane that is surrounded by the assembled protein be extruded to create a hole? To investigate the dynamics of this process, we rely on a technique called atomic force microscopy. Atomic force microscopy is the small-scale equivalent of reading Braille: With a tiny artificial finger, we feel the pore-forming proteins while they assemble on the membrane. Whereas X-ray crystallography and electron microscopy are limited to static samples, atomic force microscopy can probe active proteins while they are at work. We will thus apply atomic force microscopy to the membranes that are being exposed to attack by pore-forming proteins. Meanwhile, we will benefit from the more detailed views provided by electron microscopy to identify intermediate assemblies of pore-forming proteins, that are trapped by chemical bonds or by lowering the temperature. Electron microscopy will thus provide highly detailed pictures of pore forming proteins in different states of assembly and atomic force microscopy will enable us to see how the proteins transit between these different states.