SUT

System Under Test (SUT) is a term used in software engineering and quality assurance to denote the item being tested. The SUT is the specific software, hardware, or combination that is subjected to a test to verify that it meets specified requirements and behaves correctly under defined conditions.

In practice, identifying the SUT clarifies scope for test design, execution, and evaluation. Test cases are written to exercise the SUT’s features and interfaces, with inputs and expected outputs described in relation to the SUT. The term is used across testing levels, including unit testing (where the UUT or SUT may be a module), integration testing, system testing, and performance testing. The SUT can be a standalone application, a web service, an API, a mobile app, or an entire deployed system.

To isolate the SUT and obtain reliable results, test environments often employ stubs, mocks, or test doubles to simulate dependencies. Data setup, configuration, and hardware or network conditions are controlled to ensure reproducibility and determinism.

Limitations and risks include environmental drift, nondeterministic behavior, and incomplete coverage of external interactions. Properly defining the SUT helps align stakeholders, maintain traceability to requirements, and enable clear defect reporting.

Other uses of the acronym SUT exist in different domains, but in software practice it most commonly refers to the System Under Test.