A legacy system is an older computer system that has become too important to replace with newer technology. Old systems typically become legacy systems after excessive in-house customization has made the software incapable of being replaced or even upgraded. Legacy systems which are seriously obsolete often require the upkeep of aging mainframe hardware, which can be expensive to maintain. I.T. vendors and consultants can smell the desperation of a customer with a legacy system, which is yet another reason why it costs so much extra to maintain older hardware and software.

Legacy systems also imply that migrating to newer software would be a long and painful process, so many shops opt instead to place intermediary systems between the legacy system and their customers and staff. These intermediary systems can minimize the hassles imposed by the legacy system in a number of ways, including a telephony or web-based interface, and new software which automates otherwise tedious tasks on the legacy system. However, if the cost of keeping the legacy system alive becomes too high, most organizations will ultimately pull the plug and install something new.