Legacy System

A legacy system is an old method, technology, computer system, or application program. The legacy system may or may not remain in use. Even if it is no longer used, it may continue to impact the organization due to its historical role. Historic data may not have been converted into the new system format and may exist within the new system with the use of a customized schema crosswalk, or may exist only in a data warehouse. In either case, the effect on business intelligence and operational reporting can be significant. For a variety of reasons, a legacy system may continue to be used, sometimes well past its vendor-supported lifetime, resulting in support and maintenance challenges. It may be that the system still provides for the users' needs, even though newer technology or more efficient methods of performing a task are now available. However, the decision to keep an old system may be influenced by economic reasons such as return on investment challenges or vendor lock-in, the inherent challenges of change management, or a variety of other reasons other than functionality. A legacy system may include procedures or terminology which are no longer relevant in the current context, and may hinder or confuse understanding of the methods or technologies used.

The term "legacy" may have little to do with the size or age of the system — mainframes run 64-bit Linux and Java alongside 1960s vintage code.

Although the term is most commonly used to describe computers and software, it may also be used to describe human behaviors, methods, and tools. For example, timber framing using wattle and daub is a legacy building construction method.