A legacy system is an old or outdated method, technology, computer system, or application program that is still in use. It may refer to hardware, software applications, file formats, programming languages, or any combination of these. A legacy system may still meet the needs it was originally designed for, but its older technology may not allow it to interact with newer systems. While the term "legacy" may imply that a system is out of date or in need of replacement, a legacy system can continue to be used for a variety of reasons. Some organizations may have compelling reasons for keeping a legacy system, such as the system still providing for the users needs, economic reasons such as return on investment challenges or vendor lock-in, or the inherent challenges of change management.

However, legacy systems can cause a myriad of problems, such as exorbitant maintenance costs, data silos that prevent integration between systems, lack of compliance to governmental regulations, and reduced security. At some point, most enterprises will update or replace outdated hardware, coding language, OSes, and software. The decision to replace a legacy application or system comes down to business requirements. For many organizations, a legacy system may not necessarily need to be replaced if the technology still adequately supports its processes.

Types of legacy systems include end-of-life (EOL) legacy systems, which no longer receive updates, support, or maintenance from their software developers or creators. Legacy systems can be critical to day-to-day operations, so their migration and replacement must be carefully assessed and planned to minimize potential risks. Companies can identify they are using a legacy system or application by assessing diverse aspects, such as whether the systems vendor discontinues the product, the system is no longer updated, it is heavily patched, it is not possible to scale it, or there are no qualified staff within the company that knows how it works.

In summary, a legacy system is an outdated computing software, hardware, or application program that is still in use, but its older technology wont allow it to interact with newer systems. While legacy systems can still meet the needs of an organization, they can cause problems such as exorbitant maintenance costs, data silos, and reduced security. The decision to replace a legacy system comes down to business requirements, and migration and replacement must be carefully assessed and planned to minimize potential risks.