System integration testing

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System integration testing (SIT) involves the overall testing of a complete system of many subsystem components or elements. The system under test may be composed of electromechanical or computer hardware, or software, or hardware with embedded software, or hardware/software with human-in-the-loop testing. SIT is typically performed on a larger integrated system of components and subassemblies that have previously undergone subsystem testing.

SIT consists, initially, of the "process of assembling the constituent parts of a system in a logical, cost-effective way, comprehensively checking system execution (all nominal and exceptional paths), and including a full functional check-out."^[1] Following integration, system test is a process of "verifying that the system meets its requirements, and validating that the system performs in accordance with the customer or user expectations."^[1]

In technology product development, the beginning of system integration testing is often the first time that an entire system has been assembled such that it can be tested as a whole. In order to make system testing most productive, the many constituent assemblies and subsystems will have typically gone through a subsystem test and successfully verified that each subsystem meets its requirements at the subsystem interface level.

In the context of software systems and software engineering, system integration testing is a testing process that exercises a software system's coexistence with others. With multiple integrated systems, assuming that each have already passed system testing,^[2] SIT proceeds to test their required interactions. Following this, the deliverables are passed on to acceptance testing.^{[clarification needed]}