This article is about the legal term. For usage in the sport of cricket, see Appeal (cricket). For the African American publication "Appeal", see David Walker (who wrote it).

An appeal is the act or fact of challenging a judicially cognizable and binding decision to a higher judicial authority. In common law jurisdictions, most commonly, this means formally filing a notice of appeal with a lower court, indicating one's intention to take the matter to the next higher court with jurisdiction over the matter, and then actually filing the appeal with the higher court.

Increasingly in the United States, binding decisions can be issued in civil matters by arbitrators, referees, masters, commissioners and administrative law judges in hearings and proceedings generally classed as alternative dispute resolution. If unchallenged, these decisions have the power to settle more minor legal disputes once and for all. If one is dissatisfied with the finding of such a tribunal, one generally has the power to appeal. In some cases, the appellate step is not an appeal as such, but is known as a trial de novo. What the latter term means is that all issues and evidence may be developed newly, as though never heard before, and one is not restricted to the evidence heard in the lower proceeding. Sometimes, however, the decision of the lower proceeding is itself admissible as evidence, thus helping to curb frivolous appeals.

In an appeal from a decision in a judicial proceeding, both appellant and respondent are bound to base their arguments wholly on the proceedings and body of evidence as they were presented in the lower proceeding. Each seeks to prove to the higher court that the result they desired was the just result. Precedent and case law figure prominently in the arguments. In order for the appeal to succeed, the appellant must prove that the lower court committed reversible error that is, an impermissible action by the court acted to cause a result that was unjust, and which would not have resulted had the court acted properly. Some examples of reversible error would be permitting seriously improper argument by an attorney, admitting or excluding evidence improperly, acting outside the court's jurisdiction, injecting bias into the proceeding or appearing to do so, juror misconduct, etc. The failure to formally object at the time, to what one views as improper action in the lower court, may result in the dismissal of an appeal on the grounds that one did not "preserve the issue for appeal" by objecting.

In some rare cases, an appellant may successfully argue that the law under which the lower decision was rendered was unconstitutional or otherwise invalid, or may convince the higher court to order a new trial on the basis that evidence earlier sought was concealed or only recently discovered. In the case of new evidence, there must be a high probability that its presence or absence would have made a material difference in the trial. Another issue suitable for appeal in criminal cases is adequacy of counsel. If one faces severe penalty and can prove that he did not get a fair hearing because of incompetency on the part of his lawyer, a new trial may be forthcoming.