Short answer
These four principles are legal tests, not general invitations to decide what seems fair. They constrain EU institutions whenever they legislate or administer EU law. They also constrain national authorities when national action falls within the scope of EU law—for example, when a Member State transposes a directive or uses discretion that an EU act confers.
Proportionality asks whether a measure is suitable for a lawful objective, necessary, and, where the applicable test requires it, properly balanced against its burdens. Legal certainty requires clear, precise, and foreseeable rules. Legitimate expectations may protect reasonable reliance induced by public authorities, but rarely guarantees that regulation will never change. Equal treatment requires comparable situations to be treated alike, and different situations differently, unless an objective and proportionate justification supports the distinction.
The result turns on the governing EU measure, the authority's discretion, the burden, timing, any official assurance, and the correct comparator.
When do the principles apply?
Article 5(4) of the Treaty on European Union (TEU) expressly controls the exercise of Union competence:
Under the principle of proportionality, the content and form of Union action shall not exceed what is necessary to achieve the objectives of the Treaties. —
The Court of Justice also recognises proportionality, legal certainty, protection of legitimate expectations, and equal treatment as general principles of EU law. They therefore reach more than one Treaty article or policy field.
For national authorities, the decisive boundary is whether the conduct is governed by EU law. Article 51(1) of the Charter of Fundamental Rights states:
The provisions of this Charter are addressed to the institutions, bodies, offices and agencies of the Union with due regard for the principle of subsidiarity and to the Member States only when they are implementing Union law. —
“Implementing” is not limited to transposing a directive. It can include applying a regulation or exercising discretion EU law confers. In Plantanol, Case C-201/08, the Court applied legal certainty and legitimate expectations to a Member State using powers conferred by directives. In Duff and Others, Case C-63/93, it examined national discretion under an EU agricultural scheme against general principles and fundamental rights.
The boundary matters. The Charter does not turn every national tax, licensing, or administrative decision into an EU-law case, and Article 51(2) says that it creates no new EU power. The EU-law link must be identified first.
Proportionality connects the measure to its objective
The ordinary proportionality inquiry starts with purpose and means. Is the measure capable of advancing a legitimate objective? Does it go beyond what is necessary? If it limits a Charter right, Article 52(1) also requires a legal basis, respect for the right's essence, necessity, and a recognised general-interest objective or protection of others' rights.
The intensity of review is contextual. In Vodafone and Others, Case C-58/08, the Court accepted broad legislative discretion over complex economic choices, but still required scrutiny:
However, even though it has a broad discretion, the Community legislature must base its choice on objective criteria. Furthermore, in assessing the burdens associated with various possible measures, it must examine whether objectives pursued by the measure chosen are such as to justify even substantial negative economic consequences for certain operators. —
The Court upheld retail price caps because wholesale controls alone would not deliver the same immediate protection.
That deferential standard is not universal. Review may be stricter for a Charter-right restriction, a penalty, or an individual decision. The authority must still justify its objective, choice, and burden.
Legal certainty requires rules people can understand and anticipate
Legal certainty focuses on the rule itself and how it takes effect. In Plantanol, Germany withdrew a biofuel tax exemption before the expiry date previously stated in national legislation implementing EU directives. The Court gave the governing formulation:
the principle of legal certainty, the corollary of which is the principle of the protection of legitimate expectations, requires, on the one hand, that rules of law must be clear and precise and, on the other, that their application must be foreseeable by those subject to them —
The requirement is particularly strict for financial rules, because affected persons must determine their obligations. Publication, effective dates, conflicting provisions, retroactivity, and transition can all matter.
Foreseeability does not mean immutability. The Court said legal certainty does not prohibit legislative amendment. Instead, the legislature must consider particular situations and provide adaptations where appropriate. A clearly published prospective change with adequate time to adjust is therefore different from an abrupt, obscure, or retroactive burden.
Nor does legal certainty always require an exhaustive list. In EGB and Others, Case C-666/24, the Court of Justice held on 16 July 2026 that a national law may use abstract formulations or general categories where their interpretation remains reasonably foreseeable. The Court also confirmed that Charter equality applied because the national amnesty directly affected implementation of Directive (EU) 2017/541. The case illustrates both limits: flexible drafting still needs a workable legal meaning, and a national measure enters Charter review only through a sufficiently concrete EU-law connection.
Legitimate expectations protect justified reliance, not the status quo
Legitimate expectations are closely connected to legal certainty but ask a more specific question: did the authority create a reasonable basis for reliance?
In Plantanol, prior legislation had twice stated that the exemption would remain until a specified date, and the operator had invested. Yet early withdrawal was not automatically unlawful. The national court had to assess foreseeability, communications, publicity, transition time, prudent conduct, and the surrounding circumstances.
Precise, consistent, and attributable official representations carry more weight than silence, commercial assumptions, or general policy. A prudent operator cannot normally rely on an adjustable regulatory field remaining unchanged where change was reasonably foreseeable.
Protection need not preserve the old rule forever. Adequate notice, transition, or protection for completed transactions may answer the concern. The assurance's source, the authority's power, the reliance, and the public interest require verification.
Equal treatment begins with the correct comparator
Articles 20 and 21 of the Charter protect equality before the law and prohibit discrimination on listed and analogous grounds. The broader general principle also controls distinctions that do not involve a protected characteristic. In Arcelor Atlantique et Lorraine and Others, Case C-127/07, the Court stated:
The general principle of equal treatment, as a general principle of Community law, requires that comparable situations must not be treated differently and different situations must not be treated in the same way unless such treatment is objectively justified. —
One shared feature does not establish comparability. Relevant characteristics, the measure's purpose, and the policy field matter. Unequal treatment of comparable situations needs an objective, reasonable criterion connected and proportionate to a lawful aim.
In Arcelor, the EU emissions-trading directive initially covered steel but not certain other sectors. The Court accepted phased implementation because of the scheme's novelty and complexity, sector emissions, administrative feasibility, and legislative discretion. That does not authorise arbitrary staging: the distinction must connect to the measure's purpose through objective criteria.
How to assess a public measure in practice
A structured review can prevent the principles from being reduced to a single fairness argument:
- Establish EU-law scope. Identify the Treaty provision, regulation, directive, decision, or EU-derived power governing the authority's action.
- Define the measure and objective. Record what the authority actually did, its legal basis, and the lawful aim it invokes.
- Test proportionality. Ask about suitability, effective less burdensome alternatives, severity, duration, evidence, and the applicable intensity of review.
- Test certainty. Check wording, publication, commencement, retroactivity, conflicting rules, guidance, forms, and transition.
- Test reliance. Identify the precise representation, its authorised source, foreseeability of change, prudent conduct, investments or commitments, and possible adaptation.
- Test equality. Define the comparator by reference to the measure's purpose, identify the disadvantage, and demand an objective and proportionate justification.
- Keep the remedy separate. Illegality does not predetermine whether the result is interpretation, annulment, disapplication, repayment, damages, or another national remedy.
The same facts may matter under several headings. An abrupt distinction between two comparable licence holders may be uncertain, defeat an official assurance, and lack proportionate justification. The legal assessment should nevertheless show which test each fact satisfies.
Conclusion
Proportionality, legal certainty, legitimate expectations, and equal treatment constrain both EU institutions and Member State authorities acting within EU law. They demand a lawful and evidenced relationship between ends and means, clear and foreseeable rules, respect for justified reliance, and objective justification for distinctions between comparable situations.
They do not freeze regulation, guarantee every expectation, or require identical treatment without a valid comparator. Their force comes from disciplined questions about scope, discretion, evidence, transition, reliance, and justification. A public authority implementing EU law should be able to answer those questions from the record supporting its measure.
Sources
This article provides general legal information, not advice for a specific matter. Laws, facts and procedural deadlines may change the result. Follow the linked primary sources and obtain local advice before acting.