Fundamentals

The Principle of Legality in Criminal Law: What It Is and How It Limits Punishment

By Daniel Sardá · Published on September 24, 2026

6 min read1,275 words

The principle of legality in criminal law prevents the state from creating crimes after the fact or punishing people under unpredictable rules. Here are its safeguards and limits.

The principle of legality in criminal law rests on a simple but decisive constraint: the state cannot create a crime after someone has acted, nor impose a harsher penalty than the one applicable at that time. To punish legitimately, it needs a legal basis that existed beforehand, was public, and was sufficiently clear.

This safeguard is often expressed in the Latin formula nullum crimen, nulla poena sine lege: no crime and no punishment without law. It is not a technicality reserved for judges and lawyers. It determines whether a person could know in advance the legal consequences of an action, and it narrows the room authorities have to decide case by case what deserves punishment.

Key idea: The principle of legality does not first ask whether conduct seems blameworthy; it asks whether that conduct had already been defined as a crime and carried an applicable penalty when it occurred.

A prior rule for the power to punish

Criminal law authorizes the most intense form of state coercion: investigation, prosecution, and in some cases deprivation of liberty. That is why it is not enough for an authority to regard conduct as harmful, immoral, or undesirable. The power to punish must be subject to rules known in advance.

Article 15 of the International Covenant on Civil and Political Rights provides that no one may be convicted for acts or omissions that were not criminal when committed, or receive a heavier penalty than the one then applicable. Article 9 of the American Convention on Human Rights offers the same protection in the inter-American system.

A pre-existing rule serves two functions. For the individual, it makes it possible to guide conduct with a reasonable degree of certainty. For institutions, it sets a limit: police, prosecutors, and judges cannot devise an ad hoc criminal prohibition to resolve a case that the law did not foresee in a predictable way.

In this sense, criminal legality is a particularly demanding application of the rule of law. Authority is not placed above the rules precisely when it exercises its most severe power.

The safeguards that make legality effective

Saying that a rule must exist does not settle everything. A provision can predate the conduct and still grant nearly unlimited discretion to the person applying it. Criminal legality brings together several connected requirements:

• Prior law: the basis for the crime and its penalty must exist before the conduct occurs.
• Legislative reservation: in many civil-law systems, the creation of crimes and penalties belongs to the legislature, not to occasional administrative decisions. The precise source and scope depend on the legal system.
• Sufficient specificity: prohibited conduct and its consequence must be defined clearly enough to be reasonably knowable and to prevent authorities from having an open-ended license to punish.
• Non-retroactivity to the defendant's detriment: a later rule cannot turn past conduct into a crime or increase the penalty that applied to it.

These safeguards support one another. Prior enactment has little value if a rule is so vague that any conduct can be made to fit it; precision offers little protection if an authority can replace the rule after the fact. According to the case-law summary of the Inter-American Court of Human Rights, clear definitions of criminal offenses make it possible to distinguish punishable conduct and limit arbitrariness in the exercise of punitive power.

Key idea: It is not enough for a prohibition to appear somewhere in a text. It must impose sufficiently clear limits so that punishment does not depend on an authority's changing will.

Non-retroactivity does not mean that no later law can help

The prohibition is directed at retroactivity that harms the defendant. If a new law creates a crime or increases a penalty, it cannot be used to worsen the position of someone who acted before it entered into force.

But the treaties cited above also recognize the more favorable rule: when a later law provides a lighter penalty, the person must benefit from it. This application of the more lenient criminal law is often called lex mitior.

There is no contradiction between these rules. One prevents the state from retrospectively changing the conditions to a person's disadvantage; the other prevents a harsher punishment from being maintained once the legal system itself has reduced it. Procedures, competent authorities, and concrete effects may vary across countries, so a real case requires consulting the applicable law. The general distinction is connected to the broader question of generality of law.

An example: the Friday law and Monday's conduct

Imagine that on Monday a person engages in an activity that is lawful. On Friday, a law takes effect that makes the activity a crime. The new rule may govern future conduct on the terms set by the legal system, but it cannot justify punishment for what happened on Monday.

Now suppose that the rule does not identify particular conduct, but simply prohibits “inappropriate acts.” Even if it had been published in advance, that expression would leave authorities enormous room to decide later what to pursue. The problem would not only be temporal, but also one of specificity: a person could not reasonably anticipate where criminal liability begins.

This is a teaching example, not a conclusion about a particular case or country. It shows that legality has a formal dimension—the rule must predate the conduct—and a substantive one—its content must genuinely limit discretion.

Precision is not absolute literalism

No law can anticipate every circumstance of life. Legal terms require interpretation, and judges must decide whether particular facts fall within the description of an offense. Requiring reasonable foreseeability does not mean demanding mathematical language or forbidding all interpretive development.

The difference lies between interpreting a rule within recognizable limits and expanding a crime arbitrarily and unpredictably to a person's detriment. As a comparative reference, the European Court of Human Rights guide to Article 7 explains that gradual judicial clarification can be compatible with legality when it preserves the essence of the offense and its outcome was reasonably foreseeable.

This precision also keeps related concepts distinct. General legality requires all public power to act in accordance with law; criminal legality focuses on defining and applying crimes and penalties. Due process, by contrast, concerns how a person is charged, evidence is taken, and the case is adjudicated. A prior, clear criminal law does not establish that someone committed the act, nor does it replace the right to a defense, the presumption of innocence, or the involvement of a competent judge.

Liberty under rules, not a license for impunity

The principle of legality does not eliminate the state's capacity to prosecute validly established crimes. Nor does every minor ambiguity automatically create a path to impunity. Its function is more specific: to prevent criminal coercion from resting on retrospective, indeterminate, or person-specific prohibitions.

From the perspective of individual liberty, this foreseeability preserves a space in which people can act without fearing that lawful conduct will later be redefined. Institutionally, it requires general rules and reinforces the importance of general laws in constraining arbitrary power.

Key idea: A free society does not abandon criminal law; it requires its use to be defined before the conflict arises, rather than shaped retrospectively to secure a conviction.

This safeguard alone does not ensure a just criminal system: valid evidence, a defense, independent judges, and the other procedural protections are also necessary. But it establishes the starting point. Before asking how much to punish, public power must show that it could punish under a prior and sufficiently foreseeable rule.