Integration testing is testing that combines several modules, but still falls short of exercising the entire program all at once. Integration testing usually combines a small number of modules that call upon one another.

Integration testing can be conducted bottom-up (start by unit-testing the modules that dont'call anything else, then add the modules that call those starting modules and thest the combination, then add the modules that call those, and so on until you are ready to test main().) or top-down (start by unit-testing main() with stubs for everything it calls, then replace those stubs by the real code, but leaving in stubs for anything called from the replacement code, then replacng those stubs, and so on, until you have assembled and tested the entire program).