Over the past years, the three-dimensional structures of several bacterial porins have been determined to high resolution. Apart from revealing an unusual type of architecture, the hollow beta-barrel, they have made it possible to investigate in detail various structure-function relationships. Characteristics of ion flow through (native and modified) porins inserted into artificial bilayers have been related to the electrostatic properties of the pores. The structural basis of voltage induced pore closing, however, is still not resolved. The remarkable ability of maltoporin to allow translocation of long maltodextrin molecules through the small channel has been traced back to the presence of an elongated hydrophobic patch at the channel lining.