Publisher Summary The “narrow” in refers to the condition in which the diameter of the channel is so small that the molecules or ions in the channel are not able to pass around each other. The no-pass condition gives rise to two different sets of special effects in narrow channels. First, it provides a physical and mathematical constraint that allows the derivation of some general “thermodynamic” type of relations. The three most important relations discussed in this chapter are relation between the flux ratio, the number of ions in the channel, and the dependence of the streaming potential and of the ratio of the osmotic to diffusive water permeability on the number of water molecules in the channel. The second set of results is concerned with the implications of the no-pass condition for the general kinetic description of transport in narrow channels. Two examples discussed in the chapter are the possibilities (1) that the local mobility of an ion may depend primarily on the mobility of the chain of water molecules in the channel that it must push, rather than on its own ion-wall interaction, and (2) that one ion cannot move in the channel without displacing all other channel ions.