Delivering a new release of Windows includes a major effort to insure that applications continue to function as well on the new release as they have on the previous release. At the PDC we talked about some of the new areas of Windows Vista that reduced this level of compatibility, such as changes we made around the OS security model. With Windows 7 we renewed our engineering efforts to maintain compatibility. As with device testing, compatibility testing is an effort that spans the entire engineering organization, though we also have a group that is dedicated to this effort. This post was authored by a set of folks and coordinated by Grant George, the corporate vice president for testing in the Windows Experience team. –Steven

We aren’t going to copy the whole thing from the Engineering Windows 7 Blog,you can read about it here