Black box and white box describe how much the tester knows about the internal workings of what they are testing. Grey box sits in between, blending both.
Black Box Testing
- No knowledge of internal code.
- Tests inputs and expected outputs.
- Focuses on behaviour and requirements.
- Typical of system and acceptance testing.
White Box Testing
- Full knowledge of internal code.
- Tests paths, branches, and logic.
- Focuses on code coverage.
- Typical of unit testing by developers.
Grey box: the middle ground
Grey box testing uses partial knowledge of the internals — for example, knowing the database schema or API structure — to design smarter black box tests. It is common in integration and API testing, where understanding the architecture helps target the tests that matter.
Rule of thumb: white box answers "is every path exercised?"; black box answers "does the user-facing behaviour match the requirement?". Most products need both perspectives.
Conclusion
Black box testing validates behaviour from the user's viewpoint; white box testing validates the internal logic from the developer's viewpoint. Choosing the right mix — often with grey box in between — gives coverage that is both deep and meaningful.