Landmark court decisions, in present-day common law legal systems, establish precedents that determine a significant new legal principle or concept, or otherwise substantially affect the interpretation of existing law. "Leading case" is commonly used in the United Kingdom and other Commonwealth jurisdictions instead of "landmark case", as used in the United States. In Commonwealth countries, a reported decision is said to be a leading decision when it has come to be generally regarded as settling the law of the question involved. In 1914, Canadian jurist Augustus Henry Frazer Lefroy said "a 'leading case' [is] one that settles the law upon some important point". A leading decision may settle the law in more than one way. It may do so by:

Landmark decisions in Australia

Decisions in leading cases in Australia have usually been made by the High Court of Australia, although historically some have been made by the Judicial Committee of the Privy Council in London.

Landmark decisions in Canada

There is no universally agreed-to list of "leading decisions" in Canada. One indication, however, as to whether a case is widely regarded as being "leading" is its inclusion of the ruling in one or more of the series of compilations prepared over the years by various authors. One of the earlier examples is Augustus Henry Frazer Lefroy's Leading Cases in Canadian Constitutional Law, published in 1914. More recently, Peter H. Russell and a changing list of collaborators have published a series of books, including: Decisions in leading cases in Canada have usually been made by the Supreme Court of Canada. Prior to the abolition of appeals of Supreme Court decisions in the 1940s, most landmark decisions were made by the Judicial Committee of the Privy Council in London.

Landmark decisions in India

The Supreme Court of India, which is the highest judicial body in India, has decided many leading cases of Constitutional jurisprudence, establishing Constitution Benches for hearing the same. Given below are a list of some leading cases:

Landmark decisions in Italy

The criminal case against the operator of the Italian fake review business PromoSalento in 2018 has been described as a "landmark ruling".

Landmark decisions in New Zealand

Decisions in leading cases in New Zealand were made by the Court of Appeal of New Zealand before the establishment of the Supreme Court of New Zealand, although historically some have been made by the Judicial Committee of the Privy Council in London.

Landmark decisions in the United Kingdom

Decisions in leading cases in the United Kingdom have usually been made by the House of Lords, or more recently the Supreme Court of the United Kingdom; in Scotland by the Court of Session or High Court of Justiciary; in England and Wales by the Court of Appeal or the High Court of Justice of England and Wales.

Landmark decisions in the United States

Landmark cases in the United States come most frequently (but not exclusively) from the Supreme Court of the United States. United States Courts of Appeals may also make such decisions, particularly if the Supreme Court chooses not to review the case, or adopts the holding of the court below. Although many cases from state supreme courts are significant in developing the law of that state, only a few are so revolutionary that they announce standards that many other state courts then choose to follow.

International courts