Lex retro non agit

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Lex retro non agit means "law isn't retroactive". This rule is one of the basic rules in most countries' system of law, especially in the penal law, although there are various exceptions to this rule.

The doctrine of law unequivocally disapproves of breaking this rule. However there are some situations when legislator would want the law to work retroactively. Retroactive laws are controversial because it infringes upon individuals autonomy, and because it leads to legal uncertainty. For example, in many post-communist countries (Poland, Russia) there are disputes about imposing more severe penalties for crimes committed during the communist era (especially crimes committed by the government agents which were often acquitted or faced penalties deemed now too light). Another example of transgression of lex retro non agit rule are the rehabilitation laws, which work the other way around, annulling former penalties. But one of law functions is protecting citizens against a state, so that’s usually viewed as a non-controversial and acceptable exception.