this paper gives an overview on the structure, chemistry, self-assembly, and technical application potentials of crystalline bacterial cell surface layers (s layers). these structures represent ideal model systems for studying self-assembly processes on the nanometer level. since s layers reveal no pore size distribution they can be used for the production of a completely new type of ultrafiltration membrane. chemical modification allows specific adaption of the s-layer surface either for ultrafiltration purposes or for coating with monolayers of biologically active molecules (e.g. enzymes). as supports for covalent attachment of macromolecules, s-layer membranes might be usable for specific and rapid determination of substances by enzymatic or immunogenic methods. s-layer structures can also be used as supports for langmuir-blodgett films. finally it can be expected that as the architecture and function of more complex biological systems, particularly membranes, become better known, novel concepts will influence the design and fabrication of molecular machines and electronic devices