The maxim states that there can be no crime committed, and no punishment meted out, without a violation of penal law as it existed at the time. Another consequence of this principle is that only those penalties that had already been established for the offence in the time when it was committed can be imposed. Thus, not only the existence of the crime depends on there being a previous legal provision declaring it to be a penal offense (nullum crimen sine praevia lege), but also, for a specific penalty to be imposed in a certain case, it is also necessary that the penal legislation in force at the time when the crime was committed ranked the penalty to be imposed as one of the possible sanctions to that crime (nulla poena sine praevia lege).
Since the Nuremberg Trials, penal law is taken to include the prohibitions of international criminal law, in addition to those of domestic law. Thus prosecutions have been possible of such individuals as Nazi war criminals and officials of the German Democratic Republic responsible for the Berlin Wall, even though their deeds may have been allowed or even ordered by domestic law. Also, courts when dealing with such cases will tend to look to the letter of the law at the time, even in regimes where the law as it was written was generally disregarded in practice by its own authors.
However, some legal scholars criticize this, because generally, in the legal systems of Continental Europe where the maxim was first developed, "penal law" was taken to mean statutory penal law, so as to create a guarantee to the individual, considered as a fundamental right, that he would not be prosecuted for an action or omission that was not considered a crime according to the statutes passed by the legislators in force at the time of the action or omission, and that only those penalties that were in place when the infringement took place would be applied. Also, even if one considers that certain actions are prohibited under general principles of international law, critics point out that a prohibition in a general principle does not amount to the establishment of a crime, and that the rules of international law also do not stipulate specific penalties for the violations.
In an attempt to address those criticisms, the statute of the recently established International Criminal Court provides for a system in which crimes and penalties are expressly set out in written law, that shall only be applied to future cases.
This principle is enshrined in several national constitutions, and a number of international instruments. See e.g. European Convention on Human Rights, article 7(1); Rome Statute of the International Criminal Court, articles 22 and 23 (see )
For discussion of these issues, see Streletz, Kessler, and Krenz v. Germany (European Court of Human Rights), and K.H.-W. v. Germany (European Court of Human Rights).
The maxim itself is sometimes rendered:
or abbreviated to: