Black Hand, officially Unification or Death, was a secret society founded in the Kingdom of Serbia on May 9, 1911. as part of the Pan-Slavism nationalist movement, with the intention of uniting all of the territories containing South Slav populations annexed by Austria-Hungary.

This society's possible connections to the June 28, 1914 assassination in Sarajevo of Franz Ferdinand, Archduke of Austria is considered to have been the main catalyst to the start of World War I.

The Black Hand was founded by former members of a semi-secret society named Narodna Odbrana (Defence of the People) dedicated to achieving Pan-Slavism and nationalism by means of assassination. The purpose of the group was to recruit and train partisans for a possible war between Serbia and Austria and eventually free Serbia from Austria. Under their anti-Austrian propaganda, they organized spies and saboteurs to operate within the empire's provinces.

Satellite groups were formed in Slovenia, Bosnia, Herzegovina and Austria. In 1909, after the Bosnian Annexation Crisis, Austria pressured the Serbian government to put a stop to their anti-Austrian insurrection. At that time Russia lacked military strength to fully support Serbia in case of a war, so the organization was forced to stop.

From then on, Narodna Odbrana attempted to disguise itself as a cultural organization by shifting its concentration to education and propaganda within Serbia. In 1912, differences between the two main groups of the Narodna Odbrana—political leaders of the Radical Party and military officers—arose. The political leaders preferred a more passive approach for the time being, including more peaceful relations with Austria and concentrating on strengthening Serbia for future struggle, but some of the military officers grew impatient with the more moderate radical policies.

Consequently, the more zealous members of the Narodna Odbrana started a new secret society, and the Black Hand was founded.