Serial numbers are valuable in quality control, as once a defect is found in the production of a particular batch of product, the serial number will quickly identify which units are affected. Serial numbers are also used as a deterrent against theft in that serial numbers can be recorded and stolen goods can be identified.
Many computer programs come with serial numbers, often called "CD keys," and the installers often require the user to enter a valid serial number to continue. These numbers are verified using a certain algorithm to avoid usage of counterfeit keys. Some warez organizations supply lists of known valid serial numbers. Others will often develop a "keygen"; a program that creates a random serial number by running the installer's verification algorithm in reverse, fooling it into allowing the user to install the software without a valid licence.
Serial numbers also help track down counterfeit currency, because in some countries each banknote has a unique serial number.
See also: unique identifier