Fair Warning

Fair warning is the requirement that criminal statutes define crimes with enough precision for reasonable people to understand what activities are prohibited. According to Justice Oliver Wendell Holmes in the U.S. Supreme Court case entitled, McBoyle v. United States, 283 U.S. 25, a criminal statute must give "fair warning … in language that the common world will understand, of what the law intends to do if a certain line is passed. To make the warning fair…the line should be clear." (1931). Although ignorance of the law will generally not be a valid defense, the due process clauses that are contained in the 5th and 14th Amendments require that laws be precise enough to define what makes conduct criminal. Fair warning is also referred to as the fair warning defense or fair notice.