The Danube rises in two sources (the Brege and Brigach rivers) in the Black Forest, SW Germany, and flows NE across S Germany past Ulm to Regensburg, where it turns SE to enter Austria at Passau. It continues SE through Upper and Lower Austria, past Linz and Vienna. It then forms the border between Slovakia and Hungary from Bratislava to Szob. At Szob, the Danube turns south and flows across the Great Alföld (plain) of central Hungary, past Budapest. After forming the northern two thirds of the Croatia-Serbia border, it enters Serbia above Belgrade, turns southeast, then east, and flows through narrow gorges, forming part of the Serbia-Romania border. The Iron Gate gorge, site of a hydroelectric dam, is there; the Sip Canal bypasses rapids in the gorge. After passing the Iron Gate, the Danube broadens again and forms most of the Romania-Bulgaria border before swinging north near Silistra and passing through E Romania to Galaţi, where it divides into an expansive (c.1,000 sq mi/2,590 sq km) delta before entering the Black Sea. The northernmost branch of the delta runs along the frontier between Romania and Moldova and Ukraine. The central, canalized branch is the main shipping route. The Danube receives more than 300 tributaries, notably the Inn, Drava, Tisza, Sava, and Prut.

Navigable by barges from Ulm (by larger craft from Regensburg), the Danube is an important artery; in volume, however, Danubian commerce is far below that of the Rhine. The Danube is linked to the Main and Rhine rivers by the Rhine-Main-Danube Canal; other canals link it with the Oder and Tisza rivers. Navigation is impeded by ice in winter and by seasonally varying water levels. Hungary and Czechoslovakia (succeeded by Slovakia) agreed in 1977 to develop hydroelectric projects, which have been pursued amid controversy. Pollution of the Danube has diminished once-rich fishing grounds and rendered the water unfit for drinking and most irrigation; cleanup has proceeded slowly. Romanian efforts in the 1980s to drain land in the delta for agriculture damaged Europe's largest wetlands, which are now being rehabilitated. During the Kosovo crisis in 1999, NATO air strikes destroyed bridges across the river in Serbia, obstructing river commerce; the debris was completely cleared in 2003.

Under the Roman Empire (when it was known as Danubius and, in its lower course, as Ister), the Danube was the northern border against the barbarian world. As Rome declined, the Danubian plains for centuries attracted invading hordes—Goths, Huns, Avars, Magyars, Pechenegs, Cumans, Mongols, and others. The Danube increased in commercial importance in the era of the Crusades, but commerce suffered (15th–16th cent.) after the Turks gained control of its course from the Hungarian plain to the Black Sea. In the 19th cent. the Danube's economic importance as an international waterway increased. At the end of the Crimean War the Congress of Paris appointed (1856) a commission to clear the delta (below Brăila) of obstructions.

By the Treaty of Versailles (1919) the Danube was internationalized and a commission established with jurisdiction over the course from Ulm to Brăila. Germany repudiated the internationalization in 1936 and in 1939–40 forced both the navigation and international commissions to dissolve. After World War II, delegates from Czechoslovakia, Hungary, Yugoslavia, Romania, Bulgaria, the Soviet Union, the United States, Great Britain, and France met (1948) to determine the status of the Danube. When a commission representing only the seven riparian nations was established, the three Western nations refused to sign the convention. Subsequently, the riparian nations established a new Danube commission, based at Budapest; present membership includes Austria, Bulgaria, Croatia, Germany, Hungary, Moldova, Romania, Russia, Serbia, Slovakia, and Ukraine.