What Is a Legacy System?

by Andrea Stein

Computer systems represent a constantly evolving technology. New systems and methods are continually being devised to streamline, improve or create applications and programs. A legacy system, on the other hand, refers to old computer-related methods and technologies.


A legacy system is an old application program, computer system or other computer-related process that is still in use, despite the availability of more efficient processes or technologies. A legacy system most commonly refers to computer-related methods, although it can also be used in relation to other outdated tools, methods or human behavior.


A legacy system may reference terms or procedures which are no longer relevant in the current context of the system and can therefore lead to confusion or otherwise hinder progress. Legacy systems may run on obsolete and/or slow hardware. Obtaining parts for legacy computers can prove difficult due to obsolescence. Integrating a new system with a legacy system may also be difficult due to software incompatibilities.


A legacy system may be kept in service due to the high cost involved in replacing or redesigning the system. A legacy system may also require constant availability, such as air traffic control systems and power grids, which prohibit easily taking the legacy system out of service to redesign an up-to-date system.

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