What is STLC (Software Testing Life Cycle)?

STLC gives testing the same kind of structure the SDLC gives development. The phases are: requirement analysis (understand what to test and assess testability), test planning (scope, strategy, resources, schedule), test case design (write cases and prepare test data), test environment setup (stand up the environment and smoke-test it), test execution (run cases, log defects, retest and regress), and test closure (confirm exit criteria, archive artifacts, capture metrics and lessons learned).

The important nuance, and the interview-winning one, is that each phase has entry and exit criteria, the conditions to start and to be considered done. That is what stops a team from, say, executing tests against a broken build or declaring testing complete with critical defects open.

STLC runs in parallel with the SDLC rather than only after it, in modern, shift-left teams, testing activities start as early as requirement analysis, not at the end. Reciting the phases is fine; explaining the entry/exit criteria and the shift-left overlap with development is what shows depth.

Concrete entry and exit criteria are what interviewers listen for, so anchor each phase with an example. Requirement analysis enters when requirements are baselined and exits with a testability review and the skeleton of a requirements traceability matrix (RTM). Test planning exits with an approved test plan covering scope, risks, estimates, and the exit criteria for execution itself. Test design exits when cases are peer-reviewed and test data is prepared, environment setup exits on a passing smoke test, execution exits when planned cases have run and defect thresholds are met (for example, zero open Critical defects), and closure exits with the test summary report, metrics such as defect density and escape rate, and archived artifacts.

In agile and continuous testing, the six phases do not disappear; they compress into every sprint and even every pull request. Requirement analysis happens in backlog refinement and three-amigos sessions, planning shrinks to a lightweight test approach per story, design and execution interleave as automation is written alongside the code, environments become ephemeral (spun up per pipeline run with Docker or preview deployments), and closure becomes sprint-review metrics and dashboards instead of a formal document. The entry/exit thinking survives as CI quality gates: a merge cannot proceed unless the suite passes, coverage holds, and no blocker defects are open.

Interviewers rarely stop at naming the phases. Expect follow-ups such as: what are the entry and exit criteria of test execution, what artifact does each phase produce (test plan, RTM, test summary report), how STLC differs from SDLC, and how you would run the STLC inside a two-week sprint. Weak candidates recite six phase names; strong candidates attach one criterion, one artifact, and one agile adaptation to each phase, which proves they have operated the process rather than memorized it.