|This article does not cite any references or sources. (December 2009)|
Opinions are usually published at the direction of the court, and to the extent they contain pronouncements about what the law is and how it should be interpreted, they reinforce, change, establish, or overturn legal precedent. If a court decides that an opinion should be published, the opinion is included in a volume from a series of books called law reports (or reporters in the United States). Published opinions of courts are also collectively referred to as case law, which is one of the major sources of law in common law legal systems.
Memorandum opinion 
||The examples and perspective in this article may not represent a worldwide view of the subject. (April 2011)|
Not every case decided by a higher court results in the publication of an opinion; in fact most cases do not, since an opinion is usually only published when the law is being interpreted in a novel way, or the case is a high-profile matter of general public interest and the court wishes to make the details of its ruling public. In the majority of American cases, the judges issue what is called a memorandum decision that indicates how state or federal law applies to the case and affirms or reverses the decision of the lower court. A memorandum decision does not establish legal precedent or re-interpret the law, and cannot be invoked in subsequent cases to justify a ruling. Opinions, on the other hand, always establish a particular legal interpretation.
Legal opinions of counsel or government law officers 
In the United Kingdom and other common law countries, a legal opinion also refers to written legal advice on a point of law issued by either a barrister or advocate (often referred to as "counsel's opinion") or occasionally a senior government law officer, such as an attorney general.
The latter form of opinion is sometimes made available to the public either because of public pressure (see for example Lord Goldsmith's opinion on the Iraq War, Yoo memo), or because a general clarification of the law is called for (see for example, the Yorke–Talbot slavery opinion). In the United States, several state attorneys general issue attorney general's opinions.
Several areas of commercial practice call for formal legal opinions of counsel. The Legal Aid scheme in the United Kingdom requires a legal opinion showing reasonable prospects for success before the Legal Aid board will fund any claim. Insurance policies for professional negligence will frequently require an opinion of counsel before the insurer is required to pay out on any putative claim (sometimes called a QC clause, when it must be an opinion of leading counsel).
Legal opinion in commercial transactions 
In commercial transactions, particularly in the context of financing, the lender will require an opinion as to the corporate capacity of the borrower and enforceability of the transaction documents the borrower is intended to enter into. This type of opinion is generally issued by the borrower's counsel or the lender's counsel, depending on the ethical rule governing the issuance of those opinion applicable to the jurisdiction.
See also 
- Appellate court
- Case law
- Consilia – a historic medical opinion
- Law reports or reporters
- Memorandum opinion
- R. C. A. White, I. Boussiakou Separate opinions in the European Court of Human Rights