In American law, the clear statement rule is a guideline for statutory construction, instructing courts to not interpret a statute in a way that will have particular consequences unless the statute makes unmistakably clear its intent to achieve that result.^[1] According to law professor William Popkin, such rules "insist that a particular result can be achieved only if the text…says so in no uncertain terms."^[2]

Clear statement rules are commonly applied in areas implicating the structural constitution, such as federalism, sovereign immunity, nondelegation, preemption, or federal spending with strings attached. This is especially true when there is a strong interest against implicit abridgment of traditional understandings.

Another area in which a clear statement rule operates is with regard to legislation potentially addressing the past, instead of being forward-looking as usual.^[18] Statutory retroactivity has usually been disfavored and is in many instances forbidden by the Ex Post Facto Clause of the Constitution.^[19] Therefore:

Absent a clear statement from Congress that an amendment [to a statute] should apply retroactively, we presume that it applies only prospectively to future conduct, at least to the extent that it affects substantive rights, liabilities, or duties.^[20]

As the Supreme Court has explained,^[19] "a requirement that Congress first make its intention clear helps ensure that Congress itself has determined that the benefits of retroactivity outweigh the potential for disruption or unfairness." Such rules do, therefore, have some life in the area of substantive rights as well as enforcement of constitutional structure.

In Morrison v. National Australia Bank (2010) the Supreme Court established a presumption against extraterritorial effect and so Congress must clearly express it for U.S. laws to have effect outside U.S. boundaries.

According to Professor Popkin, Chief Justice John Marshall imposed a clear statement rule: "where fundamental values were at stake, statutes would not be interpreted to impair such values, absent a clear statement in the legislation.”^[2] Indeed, Marshall wrote in 1805 that "Where fundamental principles are overthrown, when the general system of the laws is departed from, the legislative intention must be expressed with irresistible clearness to induce a court of justice to suppose a design to effect such objects."^[21]

Marshall also required a clear statement rule to protect international law and wrote that “an act of Congress ought never to be construed to violate the law of nations if any other possible construction remains.” The Charming Betsy Doctrine is from Marshall’s opinion in Murray v. The Charming Betsy (1804), and Marshall applied a similar principle even earlier, in Talbot v. Seeman (1801).

American courts do not apply clear statement rules in all areas, however. In many cases, the court has found "implied" prohibitions and causes of action in statutes, a result that would be precluded (or at least hampered) by clear statement rules. For example, Title IX of the Education Amendments of 1972 prohibits gender discrimination by recipients of federal education funding. Does the bare prohibition also provide an implied cause of action to an individual subject to discrimination? Yes, the U.S. Supreme Court held in Cannon v. University of Chicago, 441 U.S. 677 (1979) that it provides an implied cause of action. Does the statutory prohibition on discrimination also imply a prohibition on and cause of action for,retaliation against someone who complains of such discrimination? Yes, the Court so held in Jackson v. Birmingham Board of Education, 544 U.S. 167 (2005).

Similarly, the Age Discrimination in Employment Act of 1967 (ADEA) prohibits age discrimination. Does that also imply a prohibition on retaliation against someone who complains of such discrimination? Yes, the Supreme Court held in Gomez-Perez v. Potter, 128 S. Ct. 29 (2008) that a clear statement was unnecessary to prohibit retaliation of that kind.