The Reinheitsgebot (German pronunciation: "Rine-hites-ge-boat", literally "purity order") was an ordinance brought forth by Duke Wilhelm IVan in the city of Ingolstadt in the duchy of Bavaria on April 23, 1516. The ordinance sought to restrict which grains could be used in beer production. The restriction, partially intended to prevent price competition between bakers and brewers, sought to ensure the availability of affordable bread by restricting brewer's access to more valuable wheat and rye.
In the original text, the only ingredients that could be used in the production of beer were water, barley, and hops. Violation of the ordinance could result in dumping of any beers in question.
Unfortunately for the Bavarian brewing community, and the larger German brewing community as enforcement of the Reinheitsgebot spread, this law lead to the extinction of many brewing traditions and local beer recipes, specifically, northern German spiced and cherry beers.
The Reinheitsgebot, sometimes called the "German Beer Purity Law" or the "Bavarian Purity Law" in English, has an interesting history and became a factor in the unification of Germany and the prevalence of Pilsener style beers in the German beer market.
The law was repealed by a European court in May 1987.