Normally, we think of communications science as dealing with the contemporaneous exchange of information among distant parties. Though, many of the same techniques employed in data communications are also applied to data storage to make sure that the retrieval of information from a storage medium is accurate. We find, for example, that same kinds of error-correcting codes used to protect digital telephone transmissions from noise are also used to guarantee correct readback of digital data from compact audio disks, CD-ROMs, and tape backup systems.