The inside of a cell is separated from the outside world by a cell membrane. However, stuff still needs to get in and out, and special molecules that traverse the membrane help accomplish this. A class of especially interesting molecules are the ion channels, which act like flexible tunnels that conduct ions through the cell membrane. There exists a wealth of experimental data on ion channels and even a few still pictures, but the details of the motions still are very sketchy. Biological scientists would like to understand, for example, how ion channels selectively conduct certain ions. Also, there are other ion channels that are activated by stretching the cell membrane; these are important in the function of the heart. Computer models that accurately reproduce the motions of the atoms in the channels might help us to understand the mechanisms behind these interesting observations.