A legacy system is an existing computer system or application program which continues to be used because the user (typically an organization) does not want to replace or redesign it. Many people use this term to refer to "antiquated" systems.

Legacy systems are considered to be potentially problematic by many software engineers (for example, see Bisbal et al., 1999) for several reasons. Legacy systems often run on obsolete (and usually slow) hardware, and sometimes spare parts for such computers become increasingly hard to obtain. These systems are often hard to maintain, improve, and expand because there is a general lack of understanding of the system. The designers of the system may have left the organization, so there is no one left to explain how it works. Such a lack of understanding can be exacerbated by inadequate documentation or manuals getting lost over the years. Integration with newer systems may also be difficult because new software may use completely different technologies.

Despite these problems, organizations can have compelling reasons for keeping a legacy system, such as:

• The costs of redesigning the system are prohibitive because it is large, monolithic, and/or complex.
• The system requires close to 100% availability, so it cannot be taken out of service, and the cost of designing a new system with a similar availability level is high.
• The way the system works is not well understood. Such a situation can occur when the designers of the system have left the organization, and the system has either not been fully documented or such documentation has been lost.
• The user expects that the system can easily be replaced when this becomes necessary.
• The system works satisfactorily, and the owner sees no reason for changing it – or in other words, re-learning a new system would have a prohibitive attendant personal cost in lost time and money.

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