The strategy in top-down integration is look at the design hierarchy from top to bottom. Start with the high - level modules and move downward through the design hierarchy. Modules subordinate to the top modules are integrated in the following two ways:

1. Depth first Integration: In this type, all modules on major control path of the design hierarchy are integrated first. In this example shown in fig. modules 1, 2, 6, 7/8 will be integrated first. Next, modules 1, 3, 4/5 will be integrated.
2. Breadth first Integration: In this type, all modules directly subordinate at each level, moving across the design hierarchy horizontally, are integrated first. In the example shown in figure modules 2 and 3 will be integrated first. Next, modules 6,4 and 5 will be integrated . Modules 7 and 8 will be integrated last. Procedure: The procedure for Top-Down integration process is discussed in the following steps:
3. Start with the top or initial module in the software. Substitute the stubs for all the subordinate of the top module. Test the top module.
4. After testing the top module, stubs are replaced one at a time with the actual modules for integration.
5. Perform testing on this recent integrated environment.
6. Regression testing may be conducted to ensure that new errors have not appeared.
7. Repeat steps 2-4 for whole design hierarchy.