Have you recently moved from usng FrontPage as your web editor to Expression Web? Were you used to using the FrontPage Inlcude Page feature and can not longer find it on the menu in Expression Web? Never fear, it is still there at least in Version 2.0. Adding Include Page to Insert Menu will give you step-by-stp instructions as well as screenshots.
And if you are by chance still using Version 1.0 of Expression Web Pane Manager has a free addon that will give you back your Include Page option.
Learn about about using Design Time Includes in Expression Web.