Part I State Obligations and Rights Protected, Ch.II Civil and Political Rights, Art.9: Freedom from Ex Post Facto Laws

This chapter addresses Article 9 of the American Convention on Human Rights (ACHR), which enshrines the fundamental principle of criminal legality and is interpreted as a core element of criminal law in a democratic society. The principle seeks to prohibit arbitrariness in the application of criminal law and in punishments, by requiring compliance with criminal law that is pre-established, clear, and precise. As such, Article 9 specifies in the punitive area the general principle of legality at the basis of the legitimacy of power in a democratic society. Article 9 sets out three substantive norms with a view to protecting individuals against arbitrariness of a criminal conviction. These include the affirmation of legality and prohibition of retroactivity for actions or omissions committed (first sentence); the affirmation of legality and prohibition of retroactivity for penalties applied for offences committed (second sentence); and the limitation of the prohibition of non-retroactivity of the penalty when a law subsequent to the commission of the offence provides for a more lenient penalty (third sentence).