追溯法令，相对于不溯及既往原则或祖父条款，是代表对新制度或新法律被设立前已发生或已裁决的案件，以该新法进行再审判的权力。

在刑法中，追溯法令可能会令新法成立前已进行的合法活动遭到起诉，或加重已经被判决案件的被告人刑事责任（一般禁止法律如此应用）。反之遭到起诉或裁决的案件亦可能会受惠于追溯法令，而获撤销起诉或减刑（例如中国大陆和澳门）；此情况常见于废除死刑的国家或地区改为对已判死的罪犯运行终生监禁或更轻的刑罚。

An ex post facto law (Latin for "from after the action" or "after the facts") is a law that retroactively changes the legal consequences (or status) of actions that were committed, or relationships that existed, before the enactment of the law. In criminal law, it may criminalize actions that were legal when committed; it may aggravate a crime by bringing it into a more severe category than it was in when it was committed; it may change the punishment prescribed for a crime, as by adding new penalties or extending sentences; or it may alter the rules of evidence in order to make conviction for a crime likelier than it would have been when the deed was committed. Conversely, a form of ex post facto law commonly called an amnesty law may decriminalize certain acts. A pardon has a similar effect, in a specific case instead of a class of cases. Other legal changes may alleviate possible punishments (for example by replacing the death sentence with lifelong imprisonment) retroactively. Such legal changes are also known by the Latin term in mitius.