Integration testing can proceed in a number of different ways, which can be broadly characterised as top down or bottom up. In top down integration testing the high level control routines are tested first, possibly with the middle level control structures present only as stubs. Subprogram stubs were presented in Section 2 as incomplete subprograms which are only present to allow the higher level control routines to be tested. Thus a menu driven program may have the major menu options initially only present as stubs, which merely announce that they have been successfully called, in order to allow the high level menu driver to be tested.

Top down testing can proceed in a depth-first or a breadth-first manner. For depth-first integration each module is tested in increasing detail, replacing more and more levels of detail with actual code rather than stubs. Alternatively breadth-first would proceed by refining all the modules at the same level of control throughout the application. In practice a combination of the two techniques would be used. At the initial stages all the modules might be only partly functional, possibly being implemented only to deal with non-erroneous data. These would be tested in breadth-first manner, but over a period of time each would be replaced with successive refinements which were closer to the full functionality. This allows depth-first testing of a module to be performed simultaneously with breadth-first testing of all the modules.

The other major category of integration testing is bottom up integration testing where an individual module is tested from a test harness. Once a set of individual modules have been tested they are then combined into a collection of modules, known as builds, which are then tested by a second test harness. This process can continue until the build consists of the entire application.

In practice a combination of top-down and bottom-up testing would be used. In a large software project being developed by a number of sub-teams, or a smaller project where different modules were being built by individuals. The sub-teams or individuals would conduct bottom-up testing of the modules which they were constructing before releasing them to an integration team which would assemble them together for top-down testing.