In user interface design, a modal window is a child window created by a parent application, usually a dialog box, which has to be closed before the user can continue to operate the application. Modal windows are frequently an element of MDI applications. One of their purposes is to prevent the software from being operated in an ambiguous state. For example, a spreadsheet program might ask the user whether he wants to delete a specific cell or row. If the user manually moved this row around while the dialog was still open, and then answered the question with a yes, he might not be sure what to expect: Whether the new row or the old row will be deleted, or possibly none at all because of an addressing error.

Modal windows are viewed very critically in the usability community because they block the user's workflow and require him to adapt to the software, instead of the software adapting to them. They are also frequently perceived as encouraging bad programming practices, such as not properly dealing with certain use cases because "they can never happen". They are more common in the Microsoft Windows world than in that of Mac OS, Unix-derivates and the X Window System.

The anti-modal viewpoint was one of the primary guidelines for early human-interface work. Microsoft's use of Wizards for example, was seen as anathema by many human interface guidelines, although this is considered by some to be a case of not invented here syndrome.