In law, a concurring opinion is a written opinion by one or more judges of a court which agrees with the decision made by the majority of the court, but states different reasons as the basis for his or her decision. When no absolute majority of the court can agree on the basis for deciding the case, the decision of the court may be contained in a number of concurring opinions, and the concurring opinion joined by the greatest number of judges is referred to as the plurality opinion.
There are several kinds of concurring opinion. A simple concurring opinion arises when a judge joins the decision of the court but has something to add. Concurring in judgment means that the judge agrees with the majority decision (the case's ultimate outcome in terms of who wins and who loses) but not with the reasoning of the majority opinion (why one side wins and the other loses).
In some courts, such as the Supreme Court of the United States, the majority opinion may be broken down into numbered or lettered parts, and then concurring justices may state that they join some parts of the majority opinion, but not others, for the reasons given in their concurring opinion. In other courts, such as the Supreme Court of California, the same justice may write a majority opinion and a separate concurring opinion to express additional reasons in support of the judgment (which are joined only by a minority).
As a practical matter, concurring opinions are slightly less useful to lawyers than majority opinions. Having failed to receive a majority of the court's votes, concurring opinions are not binding precedent and cannot be cited as such. But concurring opinions can sometimes be cited as a form of persuasive precedent (assuming the point of law is one on which there is no binding precedent already in effect). The conflict in views between a majority opinion and a concurring opinion can assist a lawyer in understanding the points of law articulated in the majority opinion. Occasionally, a judge will use a concurring opinion to signal that he or she is open to certain types of "test cases" that would facilitate the development of a new legal rule, and in turn, such an concurring opinion may become more famous than the majority opinion in the same case. A well-known example of this phenomenon is Escola v. Coca-Cola Bottling Co. (1944).
In some jurisdictions (e.g., California), the term may be abbreviated in certain contexts to conc. opn.
Terminology at the various courts 
- At the International Court of Justice the term separate opinion is used and judges can also ad declarations to the judgment.
- The term concurring opinion is used at the Supreme Court of the United States.
- The European Court of Human Rights uses the term concurring opinion and calls both concurring and dissenting opinions separate opinions. Judges very rarely add declarations to the judgment.
- The Law Lords of the United Kingdom give each an opinion of their own. No aggregated judgment is provided.
- See, e.g., McConnell v. Federal Election Commission, 540 U.S. 93 (2003).
- See, e.g., Cheong v. Antablin, 16 Cal. 4th 1063 (1997). Justice Ming Chin's concurrence began with these words: "Obviously, I concur in the majority opinion I have authored. I write separately to state another reason to reject plaintiff's argument."
- According to Professor Frédéric Rolin, ECHR judges added declarations in only two cases: Papon v. France (25 July 2002) and Martinie v. France (12 April 2006) ("Note sous CEDH 12 avril 2006, Martinie c/ France", 18 April 2006)