Interpretative rules are rules formulated by agencies in the form of memos, handbooks, or speeches. They are often a significant source of information about how the agency will exercise its delegation of authority. Interpretative rules are not usually the product of a specific Administrative Procedure Act. Interpretative rules do not require notice and comment procedures. The rules are not strictly binding upon the agency and the agency has discretion to change interpretative rules at any time. When an interpretative rule is challenged, courts usually defer to the agency's expertise in the interpretation. However, when the agency rules are totally ultra vires, courts may substitute their judgment for that of the agency.