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The Duchy of Holstein was the northernmost state of the Holy Roman Empire. It was located in today's state of Schleswig-Holstein. The Duchy existed from 1474 to 1867. It was founded by King Christian I of Denmark, when he had his County elevated to a duchy. It was ruled jointly with the Duchy of Schleswig by members of the Danish House of Oldenburg for its entire existence.

1815 became Schleswig-Holstein a German Confederation, but the title of duke was permanent by the King of Denmark. In the First Schleswig War fought revolutionaries against the troops from Denmark, because the wanted a state with a German territorial lord. The war was between 1848 to 1851. Years later, In 1864, Holstein was again occupied by troops of the German Confederation. After the German-Danish War, Holstein came with Schleswig and Lauenburg in an Austrian-Prussian condominium. After that, it would part of the new Prussian province Schleswig-Holstein.

During the WWII came millions of refugees from Pomerania and East Prussia to Schleswig-Holstein. Like so many cities, Kiel in Schleswig-Holstein was heavily destroyed by air strikes.

In 1946, Schleswig-Holstein became a independent state, after the he crushing of Prussia and 3 years later, a country of the Federal Republic of Germany.