The core of modern potassium channels originated at least as early as bacteria.Proceeding from outside to in, the core has a narrow selectivity filter that permits only passage of dehydrated K ions, a wider vestibule through which hydrated K ions move easily, and, at its inner end, a gate for turning ion current on and off.The vestibule is a binding site for many drugs that act on channels.Later in evolution, numerous varieties of K channels developed, all with a common core and differing mainly in the signals that control the gate.Most familiar are the voltage-gated channels, which open and close in response to changes of the membrane potential.The molecular machinery for sensing the membrane voltage and locking or unlocking the gate is now reasonably well understood, and will be described.In neurons voltage-gated K channels repolarize the action potential, and play a role in spike timing.Cardiac potassium channels of several types are crucially involved in pacemaking and control of the duration of contraction.Similar functions are undoubtedly served by potassium channels in the nervous system, but are less well understood.Possible roles for K channels in short term memory, timing functions, and coincidence detection will be discussed.Elsewhere in the body there are countless uses for K channels, including control of secretion and control of blood glucose.The vast majority of our cells lack electrical signaling functions, but have potassium channels which establish the resting potential, making possible osmotic balance: with lowered membrane voltage cells swell, as happens in cerebral edema.Some of these many functions will be examined in the light of advancing biophysical and structural understanding of the channnels.