In law, entry into force or entry into force[1]​ is the process by which legislation, regulations, treaties and other legal instruments") come to have the force and effect of law or, what is the same, it represents the beginning of the effectiveness of a legal norm.[2]​ The term is closely related to the date of this transition. The moment in which said instrument comes into force can be established in the document itself, after the passage of a certain period or upon the occurrence of a certain event, such as a proclamation or an objective event (for example, the birth, marriage, reaching a certain age or death of a certain person).[3]​ In rare cases, the legal effects of a law can be retroactive to a date before the promulgation.[4]​.

To come into force, a law or treaty must first receive the required number of votes or ratifications. While it is common practice to stipulate this number as a requirement in the text of the treaty itself, it may also be set out in a higher law or legal framework, such as a constitution or the internal regulations of the legislature in which it originated.[5]​.

Typically, the process by which a bill becomes law includes signature or some other form of consent by the head of state and publication in an official journal, so that the population knows it exists and is aware of the law or treaty. In some systems, the head of state or some other authorized official is required to definitively express his approval, as for example in the granting of royal assent to governors-general in Commonwealth realms.[6]​ In others, a bill automatically becomes law unless it is vetoed (for example, by the president) or sent for judicial review (if an appeal for unconstitutionality has been brought to the supreme court or constitutional court).[7]​[8]​.

After their adoption, treaties, as well as their amendments "Amendment (Law)"), may have to follow the official legal procedures of the international organization that sponsored them, including signature, ratification and entry into force.[9]​.