Fifteen years ago the presence of ion channels in the cytoplasmic membrane of bacteria was held by many to be inherently unlikely, given the prominent role played by the membrane in energy transduction. The presence of channels in the outer membrane of Gram negative bacteria was known and the physiological roles, structures and genetics were already well-established (1). A number of studies subsequently demonstrated pressure-sensitive channels in a range of Gram positive and Gram negative bacteria (2-4), but the cloning of the first structural gene for a mechanosensitive channel, MscL, with the attendant careful characterization (5) finally silenced the doubters. The general acceptance of bacterial ion channels owes much to two events in the 90’s. The first was the cloning and sequencing of the structural genes for mammalian channels, which were used to demonstrate the presence of a specific K+channel inStreptomyces lividans( 6). Subsequently, the completion of the genome sequences for many microorganisms demonstrated the presence of several classes of channels in a range of bacteria, archaea and yeasts (7-8). The second seminal event was the exploitation of the bacterial channel genes to provide large quantities of the proteins for biophysical and crystallographic analysis (9-13). Bacterial channel proteins have provided structural paradigms for four major classes of channels (five if the porin channels are included; 14) and have become the test-bed for ideas about channel structure and gating that could not readily be tested in higher eukaryotes.