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.
In 1914, Canadian jurist Augustus Henry Frazer Lefroy said "a 'leading case' [is] one that settles the law upon some important point".
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.