Integration testing should expose defects in the interfaces, and the interaction between integrated components or modules. There are different strategies for performing integration testing. These strategies describe the order in which units are to be integrated, presuming that the units have been separately tested. Examples of common integration strategies are the following:

• Top-down integration: This strategy starts with the main unit (module), that is, the root of the procedural tree. Any lower-level module that is called by the main unit should be substituted by a test double. Once testers are convinced that the main unit logic is correct, the stubs are gradually replaced with the actual code. This process is repeated for the rest of the lower-unit in the procedural tree. The main advantage of this approach is that defects are more easily found.
• Bottom-up integration: This strategy starts the testing process with the most elementary units. Larger subsystems are assembled from the tested components. The main advantage of this type is that test doubles are not needed.
• Ad hoc integration: The components are integrated in the natural order in which are finished. It allows an early testing of the system. Test doubles are usually required.
• Backbone integration: A skeleton of components is built and others are gradually integrated. The main disadvantage of this approach is the creation of the backbone, which can be labor-intensive.