Legalization is an act to make a thing, situation or practice lawful. Legalization is the final step in the regularization process whereby an appropriate public authority removes a legal prohibition against something, a situation or practice that is previously not legal.

In another context, legalization can be a procedure for confirming that a document issued by the authorities of a state, or drawn up with the participation of these authorities, complies with the legislation of that state and creates associated legal rights and obligations.

However, such formal processes and procedures are also governed by general principles of international law, including peremptory norms, customary law and provisions of both public and private international law. Non-derogable human rights also prohibit certain acts of law that would arbitrarily deprive a person of her/his life; subject a person to torture, or to cruel, inhuman or degrading treatment or punishment; hold a person in slavery or servitude; imprison someone merely on the ground of inability to fulfill a contractual obligation; hold a person guilty of any criminal offence on account of any act or omission that did not constitute a criminal offence, under national or international law, at the time when it was committed; deny recognition anyone anywhere as a person before the law; or deny the right to freedom of thought, conscience and religion (ICCPR), articles 6, 7, 8 (paragraphs I and 2), 11, 15, 16 and 18).

Generally, common-law jurisdictions do not permit retroactive criminal legislation, although new judicial precedent can apply to events that occurred before the court decision. Ex post facto federal and state laws are expressly forbidden by the United States Constitution. In a nation with an entrenched bill of rights or a written constitution, ex post facto legislation may be prohibited or allowed. For example, some legal systems explicitly allow retroactive effect for laws that reduce possible punishments.

Ex post facto criminalization is prohibited by the European Convention on Human Rights (Article 7), International Covenant on Civil and Political Rights (Article 15(1)), and the American Convention on Human Rights (Article 9). Jurisdictions of American states generally prohibit ex post facto laws.

The same conditions also apply to treaties. The Vienna Convention on the Law of Treaties provides that, if a treaty conflicts with a peremptory norm of general international law (jus cogens), it is void (Article 53). It also provides that, in the case of a state’s internal law, no such legislation is permitted as justification for the state’s failure to perform a treaty (Article 26).