Crystalline cell surface layers (S-layers) represent the outermost cell envelope component in many bacteria. The oblique, square or hexagonal lattices are formed of assemblies of a single protein or glycoprotein species. Isolated S-layer subunits are endowed with the ability to assemble into monomolecular arrays in suspension, on solid surfaces (e.g. metals, polymers, glass, carbon, silicon), at the air/water interface, or on lipid films generated by the Langmuir Blodgett technique. S-layer lattices are isoporous structures with functional groups located on the surface and in the pores in an identical position and orientation. These characteristic features have led to applications of S-layers as (i) ultrafiltration membranes with pores of identical size and morphology and a broad chemical modification potential, (ii) matrices for the controlled and reproducible immobilization of functional macromolecules, as required for affinity and enzyme membranes, affinity microcarriers and biosensors, (iii) carriers for Langmuir-Blodgett films and reconstituted biological membranes, (iv) immobilization matrices and adjuvants for weakly immunogenic antigens and haptens and (v) patterning elements in molecular nanotechnology.