General principles of European Union law

The general principles of European Union law are general principles of law which are applied by the European Court of Justice and the national courts of the member states when determining the lawfulness of legislative and administrative measures within the European Union. General principles of European Union law may be derived from common legal principles in the various EU member states, or general principles found in international law or European Union law. Amongst others the European Court of Justice has recognised fundamental rights, proportionality, legal certainty, equality before the law and subsidiarity as general principles of European Union law. General principles of law should be distinguished from rules of law as principles are more general and open-ended in the sense that they need to be honed to be applied to specific cases with correct results.
Background.
The general principles of European Union law are rules of law which a European Union judge, sitting for example in the European Court of Justice, has to find and apply but not create. Particularly for fundamental rights, Article 6(3) of the Treaty on European Union provided:
Further, Article 340 of the Treaty on the Functioning of the European Union (formerly Article 215 of the Treaty establishing the European Economic Community) expressly provides for the application of the "general principles common to the laws of the Member States" in the case of non-contractual liability. 
In practice the European Court of Justice has applied general principles to all aspects of European Union law. In formulating general principles, European Union judges draw on a variety of sources, including: public international law and its general principles inherent to all legal systems; national laws of the member states, that is general principles common to the laws of all member states, general principles inferred from European Union law, and fundamental human rights. General principles are found and applied to avoid the denial of justice, fill gaps in European Union law and to strengthen the coherence of European Union law.
Examples.
Accepted general principles of European Union Law include fundamental rights (see human rights), proportionality, legal certainty, equality before the law and subsidiarity. In Case T-74/00 "Artegodan", the General Court (then Court of First Instance) appeared willing to extrapolate from the limited provision for the precautionary principle in environmental policy in Article 191(2) TFEU to a general principle of EU law.
Fundamental rights.
Fundamental rights, as in human rights, were first recognised by the European Court of Justice based on arguments developed by the German Constitutional Court in "Stauder v City of Ulm" Case 29/69 in relation to a European Community scheme to provide cheap butter to recipients of welfare benefits. When the case was referred to the European Court of Justice the ruling of the German Constitutional Court, the European Community could not "prejudice the fundamental human rights enshrined in the general principles of Community law and protected by the Court". This concept was further developed by the European Court of Justice in "International Handelsgesellschaft v Einfuhr- und Vorratsstelle Getreide" [1970] ECR 1125 Case 11/70 when it was held that "Respect for fundamental rights form an integral part of the general principles of law protected by the Court of Justice. The protection of such rights, whilst inspired by the constitutional traditions common to the member states, must be ensured within the framework of the structure and objectives of the Community." Subsequently, in "J Nold v Commission" Case 4/73 the European Court of Justice reiterated that human rights are an integral part of the general principles of European Union law and that as such the European Court of Justice was bound to draw inspiration from the constitutional traditions common to the member states. Therefore, the European Court of Justice cannot uphold measures which are incompatible with fundamental rights recognised and protected in the constitutions of member states. The European Court of Justice also found that "international treaties for the protection of human rights on which the member states have collaborated or of which they are signatories, can supply guidelines which should be followed within the framework of Community law."
The Charter of Fundamental Rights of the European Union.
None of the original treaties establishing the European Union mention protection for fundamental rights. It was not envisaged for European Union measures, that is legislative and administrative actions by European Union institutions, to be subject to human rights. At the time the only concern was that member states should be prevented from violating human rights, hence the establishment of the European Convention on Human Rights in 1950 and the establishment of the European Court of Human Rights. The European Court of Justice recognised fundamental rights as general principle of European Union law as the need to ensure that European Union measures are compatible with the human rights enshrined in member states' constitution became ever more apparent. In 1999 the European Council set up a body tasked with drafting a European Charter of Human Rights, which could form the constitutional basis for the European Union and as such tailored specifically to apply to the European Union and its institutions. The Charter of Fundamental Rights of the European Union draws a list of fundamental rights from the European Convention on Human Rights and Fundamental Freedoms, the Declaration on Fundamental Rights produced by the European Parliament in 1989 and European Union Treaties. 
The 2007 Lisbon Treaty explicitly recognised fundamental rights by providing in Article 6(1) that "The Union recognises the rights, freedoms and principles set out in the Charter of Fundamental Rights of the European Union of 7 December 2000, as adopted at Strasbourg on 12 December 2007, which shall have the same legal value as the Treaties." Therefore, the Charter of Fundamental Rights of the European Union has become an integral part of European Union law, codifying the fundamental rights which were previously considered general principles of European Union law. In effect, after the Lisbon Treaty, the Charter and the convention now co-exist under European Union law, though the former is enforced by the European Court of Justice in relation to European Union measures, and the latter by the European Court of Human Rights in relation to measures by member states.
Legal certainty.
The concept of legal certainty is recognised 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 retroactive laws: 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" [1997] 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 and 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 the 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" [1988] ECR 2321 Case 120/86. The misuse of powers 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 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" [1976] 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.
Proportionality.
The legal concept of proportionality is recognised one of the general principles of European Union law by the European Court of Justice since the 1950s. It was first recognised by the European Court of Justice in "Federation Charbonniere de Belgique v High Authority" [1954] ECR 245 Case C8/55 and in "Internationale Handelsgesellschaft v Einfuhr- und Vorratsstelle Getreide" [1970] ECR 1125 Case 11/70 the European Advocate General provided an early formulation of the general principle of proportionality in stating that "the individual should not have his freedom of action limited beyond the degree necessary in the public interest". The general concept of proportionality has since been further developed, notably in "R v Minister of Agriculture, Fisheries and Food ex parte Fedesa" [1990] ECR 1–4023 Case C-331/88 in which a European directive prohibiting the use of certain hormonal substances in livestock farming was challenged. In its ruling the European Court of Justice held that by virtue of the general principle of proportionality the lawfulness of the Directive depended on whether it was appropriate and necessary to achieve the objectives legitimately pursued by the law in question. When there was a choice between several appropriate measures the least onerous must be adopted, and any disadvantage caused must not be disproportionate to the aims pursued. The principle of proportionality is also recognised in Article 5 of the EC Treaty, stating that "any action by the Community shall not go beyond what is necessary to achieve the objectives of this Treaty".
Prof Grainne de Burca has therefore argued that the general principle in European Union law of proportionality entails a three-part test: 1) is the measure suitable to achieve a legitimate aim, 2) is the measure necessary to achieve that aim or are less restrictive means available, and 3) does the measure have an excessive effect on the applicant's interests. The general principle of proportionality therefore requires that a measure is both appropriate and necessary, and as such the European Court of Justice to review both the legality of a measure, but also to some extent the merit of legislative and administrative measures. Therefore, the general principle of European Union law of proportionality is often considered as the most far-reaching ground of judicial review and of particular importance in public law cases. However, because the proportionality concept potentially concerns the merits of a measure, European judges may defer to the choice of the authority which has adopted the measure, or make what are frequently political decisions. In "Fedesa" the European Court of Justice explained that "it must be states that in matters concerning the common agricultural policy the Community legislature has the discretionary power which corresponds to the political responsibilities given to it by... the Treaty. Consequently, the legality of a measure adopted in that sphere can be affected only if the measure is manifestly inappropriate having regard to the objective which the competent institution is seeking to pursue...".