The Schleswig–Holstein question (German: Schleswig-Holsteinische Frage; Danish: Spørgsmålet om Sønderjylland og Holsten) was a complex set of diplomatic and other issues arising in the 19th century from the relations of two duchies, Schleswig (Sønderjylland/Slesvig) and Holstein (Holsten), to the Danish crown, to the German Confederation, and to each other. The British statesman Lord Palmerston is reported to have said: "Only three people have ever really understood the Schleswig-Holstein business – the Prince Consort, who is dead – a German professor, who has gone mad – and I, who have forgotten all about it."

Schleswig was part of Denmark during the Viking Age, and became a Danish duchy in the 12th century. Denmark repeatedly tried to reintegrate the Duchy of Schleswig into the Danish kingdom. Holstein, just on the other side of the Danish border from Schleswig, was in medieval times a fief of the Holy Roman Empire. From 1460 on, the two had been ruled together by a common Duke, who in practice was also the King of Denmark. The Treaty of Ribe, agreed to by the Danish King in order to gain control of both states, seemed to indicate that Schleswig and Holstein were to remain united, though that interpretation was later challenged.

The Holy Roman Empire was dissolved in 1806. The German Confederation, formed in 1815, included Holstein. By the early 19th century, Holstein's population was, along with much of Schleswig's, almost entirely ethnically German.

Both Schleswig and Holstein had been ruled through institutions separate from the rest of the Danish Kingdom. On March 27, 1848, Frederick VII of Denmark announced to the people of Schleswig the promulgation of a liberal constitution under which the duchy, while preserving its local autonomy, would become an integral part of Denmark. This led to an open uprising by Schleswig-Holstein's large German majority in support of independence from Denmark and of close association with the German Confederation. The military intervention of the Kingdom of Prussia supported the uprising: the Prussian army drove Denmark's troops from Schleswig and Holstein, beginning the First Schleswig War (1848–51), which ended in a Danish victory at Idstedt; with the London Protocol, the international community agreed on the Duchies' status.

A second crisis emerged due to a succession dispute. The Dukedoms of Holstein and Lauenburg were legally inherited under the German Salic law which ignored females; differing laws in the Kingdom of Denmark with Schleswig permitted male heirs to inherit through a female line. Under these varying laws, the childless King Frederick VII of Denmark would have different heirs in Denmark and in Holstein. But when Frederick died in 1863, his Danish heir Christian IX claimed to have inherited the Duchy of Holstein as well, and attempted to reintegrate the Duchy of Schleswig into the Danish kingdom by the signing the so-called November Constitution. This was seen as a violation of the London Protocol, and it led to the Second Schleswig War of 1864 and ultimately to the Duchies' absorption into Germany.

Following the defeat of Germany in World War I, the Danish-majority area of Northern Schleswig was finally unified with Denmark after two plebiscites organised by the Allied powers. A small minority of ethnic Germans still lives in Northern Schleswig, while a Danish minority remains in South Schleswig.