Grey box testing is a software testing technique that uses a combination of black box testing and white box testing. Gray box testing is not black box testing, because the tester does know some of the internal workings of the software under test. In grey box testing, the tester applies a limited number of test cases to the internal workings of the software under test. In the remaining part of the grey box testing, one takes a black box approach in applying inputs to the software under test and observing the outputs.