Any punishment must be founded in law

Article 7: No one shall be held guilty of any criminal offence on account of any act or omission which did not constitute a criminal offence under national or international law at the time when it was committed.

Article 7 of the Convention precludes any retrospective effects of laws by stating that individuals may not be found guilty of acts or omissions which did not constitute an offence when they were committed. In addition, a sentence more severe than the sentence applicable when an offence was committed may not be imposed.