Crystalline surface layers (S-layers) represent an almost universal feature of archaebacterial cell envelopes and can be found in gram-positive and gram-negative eubacterial species from nearly all phylogenetic branches. S-layers consist of a single protein- or glycoprotein species and thus can be considered as one of the most primitive membrane structures developed during evolution. Prokaryotes carrying S-layers are ubiquitously found in every part of the biosphere. This supports the concept of a general supramolecular "porous crystalline surface layer" fulfilling a broad spectrum of functions which are strongly dependent on the particular environmental and ecological conditions. Their structural simplicity makes S-layers a suitable model for analyzing structure-function relationships as well as dynamic aspects of membrane morphogenesis.