System integration is the bringing together of the component subsystems into one system and ensuring that the subsystems function together as a system. In information technology, systems integration is the process of linking together different computing systems and software applications physically or functionally.The system integrator brings together discrete systems utilizing a variety of techniques such as computer networking, enterprise application integration, business process management or manual programming.

A system is an aggregation of subsystems cooperating so that the system is able to deliver the over-arching functionality. System integration involves integrating existing (often disparate) subsystems. The subsystems will have interfaces. Integration involves joining the subsystems together by “gluing” their interfaces together. If the interfaces don’t directly interlock, the “glue” between them can provide the required mappings. System integration is about determining the required “glue”.

System integration is also about value-adding to the system, capabilities that are possible because of interactions between subsystems.