A preamble is the part of a rulemaking document, usually published in the Federal Register, that explains the reasons for the regulatory action of a Federal agency. The preamble of a proposed rule contains information about the bases for the proposal, possible hearings, contact people, comment dates, and related materials that are available to the public. The preamble of a final rule contains the statement of "basis and purpose" of a regulation as required by the Administrative Procedure Act. This also normally includes the agency's response to comments filed by the public.

A preamble is not a part of the regulatory text, and therefore does not appear in the Code of Federal Regulations. A preamble is not legally enforceable, but it is an important aid in gaining an understanding of why an agency is acting or refusing to act. A preamble is also part of the "informal record"--the material that would be reviewed by a court to determine if an agency has acted in an arbitrary or capricious manner.