Potassium channels are now known in virtually all types of cell in all organisms, where they are involved in a multitude of physiological functions. K⁺ channels are ubiquitous and critical for life. They are found in archaebacteria, eubacteria, and eukaryotic cells, both plant and animal, and their amino acid sequences are very easy to recognize because potassium channels contain a highly conserved segment called the K⁺ channel signature sequence. This sequence forms a structural element known as the selectivity filter, which prevents the passage of Na⁺ ions but allows K⁺ ions to conduct across the membrane at rates approaching the diffusion limit. The K⁺ selectivity filter catalyzes the dehydration, transfer, and rehydration of a K⁺ ion within about ten nanoseconds. This physical process is central in the production of electrical signals in biology. The selectivity filter contains two K⁺ ions about 7.5 angstroms apart. This configuration promotes ion conduction by exploiting electrostatic repulsive forces to overcome attractive forces between K⁺ ions and the selectivity filter. The architecture of the pore establishes the physical principles underlying selective K⁺ conduction.