Nullum crimen sine lege is the criminal law principle that a person cannot be convicted of an offence unless the conduct was clearly defined as a crime by law at the time it was committed. It protects legal certainty and the rule of law by prohibiting retroactive criminal liability and vague or excessively elastic interpretations of criminal offences. In the UK and Ireland it is not usually stated in Latin in statutes, but is reflected in constitutional and human rights instruments, including Article 7 of the European Convention on Human Rights, incorporated by the Human Rights Act 1998 (England & Wales, Scotland, Northern Ireland) and the Human Rights Act 1998 (NI) Order, and in Irish constitutional jurisprudence on legality and foreseeability in criminal law. Courts apply the principle when interpreting criminal statutes and common law offences, limiting judicial development of the law where this would amount to creating a new offence or extending existing offences in an unforeseeable way. Across England & Wales, Scotland, Northern Ireland and Ireland, its practical effect is broadly consistent: criminal liability must be based on pre-existing, accessible and sufficiently precise law.