Legal certainty is a principle in national and international law which holds that the law must provide those subject to it with the ability to regulate their conduct. Legal certainty is internationally recognised as a central requirement for the rule of law.
The legal system needs to permit those subject to the law to regulate their conduct with certainty and to protect those subject to the law from arbitrary use of state power. Legal certainty represents a requirement that decisions be made according to legal rules, i.e. be lawful. The concept of legal certainty may be strongly linked to that of individual autonomy in national jurisprudence. The degree to which the concept of legal certainty is incorporated into law varies depending on national jurisprudence. However, legal certainty frequently serves as the central principle for the development of legal methods by which law is made, interpreted and applied.
Legal certainty is an established legal concept both in the civil law legal systems and common law legal systems. In the civil law tradition, legal certainty is defined in terms of maximum predictability of officials' behaviour. In the common law tradition, legal certainty is often explained in terms of citizens' ability to organise their affairs in such a way that does not break the law. In both legal traditions, legal certainty is regarded as grounding value for the legality of legislative and administrative measures taken by public authorities.
Rule of law 
The legal philosopher Gustav Radbruch regarded legal certainty, justice and policy as the three fundamental pillars of law. Today legal certainty is internationally recognised as a central requirement for the rule of law. According to the Organisation for Economic Co-operation and Development (OECD) the concept of the rule of law "first and foremost seeks to emphasize the necessity of establishing a rule-based society in the interest of legal certainty and predictability." At the G8 Foreign Ministers' Meeting in Potsdam in 2007, the G8 committed to the rule of law as a core principle entailing adherence to the principle of legal certainty.
European nations regard legal certainty as a fundamental quality of the legal system and a guiding requirement for the rule of law. The concept can be traced through English common law and is recognised in all European legal systems. The concept is recognised in Germany as Rechtssicherheit, in France as sécurité juridique, in Spain as seguridad juridica, in Italy as certezza del diritto, in the Benelux countries as rechtszekerheid, in Sweden as Rättssäkerhet, in Poland as do obowiazujacego prawa, and in Finland as oikeusvarmuuden periaate. Legal certainty is now recognised as one of the general principles of European community law and "requires that all law be sufficiently precise to allow the person - if need be, with appropriate advice - to foresee, to a degree that is reasonable in the circumstances, the consequences which a given action may entail". The principle of legal certainty, and as such the rule of law, requires that:
- laws and decisions must be made public
- laws and decisions must be definite and clear
- the decisions of courts must be regarded as binding
- the retroactivity of laws and decisions must be limited
- legitimate interests and expectations must be protected.
General principle of European Union law 
The concept of legal certainty has been recognised as one of the general principles of European Union law by the European Court of Justice since the 1960s. It is an important general principle of international law and public law, which predates European Union law. As a general principle in European Union law, it means that the law must be certain, in that it is clear and precise, and its legal implications foreseeable, especially when applied to financial obligations. The adoption of laws which will have legal effect in the European Union must have a proper legal basis. Legislation in member states which implements European Union law must be worded so that it is clearly understandable by those who are subject to the law.
In European Union law the general principle of legal certainty prohibits Ex post facto laws, i.e. laws should not take effect before they are published. The general principle also requires that sufficient information must be made public to enable parties to know what the law is and comply with it. For example in Opel Austria v Council  ECR II-39 Case T-115/94 The European Court of Justice held that European Council Regulation did not come into effect until it had been published. Opel had brought the action on the basis that the Regulation in question violated the principle of legal certainty, because it legally came into effect before it had been notified and the regulation published. The doctrine of legitimate expectation, which has its roots in the principles of legal certainty and good faith, is also a central element of the general principle of legal certainty in European Union law.
The legitimate expectation doctrine holds that "those who act in good faith on the basis of law as it is, or seems to be, should not be frustrated in their expectations". This means that a European Union institution, once it has induced a party to take a particular course of action, must not renege on its earlier position if doing so would cause that party to suffer loss. The European Court of Justice has considered the legitimate expectation doctrine in cases where violation of the general principle of legal certainty was alleged in numerous cases involving agricultural policy and European Council regulations, with the leading case being Mulder v Minister van Landbouw en Visserij  ECR 2321 Case 120/86.
The misuse of power test is another significant element of the general principle of legal certainty in European Union law. It holds that a lawful power must not be exercised for any other purpose than that for which it was conferred. According to the misuse of power test a decision by a European Union institution is only a misuse of power if "it appears, on the basis of objective, relevant and consistent evidence, to have been adopted with the exclusive or main purpose of achieving any end other than those stated." A rare instance where the European Court of Justice has held that a European Union institution has misused its powers, and therefore violated the general principle of legal uncertainty, is Giuffrida v Commission  ECR 1395 Case 105/75. The general principle of legal certainty is particularly stringently applied when European Union law imposes financial burdens on private parties.
European human rights law 
See also 
- Maxeiner, James R. (Fall 2008). "Some realism about legal certainty in globalization of the rule of law". Houston Journal of International law. Retrieved 29 May 2011.
- Claes, Erik; Devroe, Wouter; Keirsblick, Bert (2009). Facing the limits of the law. Springer. pp. 92–93. ISBN 978-3-540-79855-2.
- Chalmers, Damian (2006). European Union law: text and materials. Cambridge University Press. p. 454. ISBN 978-0-521-52741-5.
- Kaczorowsky, Alina (2008). European Union law. Taylor & Francis. p. 232. ISBN 978-0-415-44797-3.
- Chalmers, Damian (2006). European Union law: text and materials. Cambridge University Press. p. 455. ISBN 978-0-521-52741-5.
- Chalmers, Damian (2006). European Union law: text and materials. Cambridge University Press. p. 456. ISBN 978-0-521-52741-5.
- Claes, Erik; Devroe, Wouter; Keirsblick, Bert (2009). Facing the limits of the law. Springer. p. 93. ISBN 978-3-540-79855-2.