The lands which comprise the modern duchy of Brunswick belonged in the 10th century to the family of the Brunos, whence the name Brunswick is derived, of the counts of Nordheim, and the counts of Supplinburg. Inherited during the 12th century by Henry the Proud, duke of Saxony and Bavaria, and a member of the family of Welf, they subsequently formed part of the extensive Saxon duchy ruled by his son, Henry the Lion.

When Henry was placed under the imperial ban and his duchy dismembered in 1181, he was allowed to retain his hereditary possessions, which consisted of a large part of Brunswick and Lüneburg. The bulk of these lands came subsequently to Henry's grandson, Otto, and in 1235 the emperor Frederick II., anxious to be reconciled with the Welfs, recognized Otto's title and created him duke of Brunswick and Lüneburg. Otto added several counties and the town of Hanover to his possessions, and when he died in 1252 was succeeded by his sons Albert and John. In 1267 these princes divided the duchy, Albert becoming duke of Brunswick, and John duke of Lüneburg. The dukes of Lüneburg increased the area of their duchy, and when the family died out in 1369 a stubborn contest took place for its possession. Claimed by Magnus II., duke of Brunswick-Wolfenbüttel, this prince was forced by the emperor Charles IV. to abandon his pretensions, but in 1388 his sons succeeded in incorporating Lüneburg with Brunswick-Wolfenbüttel. In 1285 the duchy of Brunswick had been divided between Duke Albert's three sons, whose relations with each other were far from harmonious, and the lines of Wolfenbüttel, Göttingen and Grubenhagen had been established.

The Wolfenbüttel branch died out in 1292, but was refounded in 1345 by Magnus I., a younger member of the Göttingen family; the elder Göttingen branch died out in 1463, and the Grubenhagen branch in 1596. Magnus I., duke of Brunswick-Wolfenbüttel from 1345 to 1369, was the ancestor of the later dukes of Brunswick. His grandsons, Frederick, Bernard and Henry, secured Lüneburg in 1388, but in 1428 Bernard, the only survivor of the three, was forced to make a division of the duchy, by which he received Lüneburg, while his nephews, William and Henry, obtained Brunswick, which in 1432 they divided into Calenberg and Wolfenbüttel. In 1473, however, William, who had added Göttingen to his possessions in 1463, united these lands; but they were again divided from 1495 to 1584. In 1584 Brunswick was united by Duke Julius, and in 1596 Grubenhagen was added to it. Duke Frederick Ulrich, however, was obliged to cede this territory to Lüneburg in 1617, and when he died in 1634 his family became extinct, and Brunswick was divided between the two branches of the Lüneburg family.

The duchy of Lüneburg, founded by Bernard in 1428, remained undivided until 1520, when Duke Henry abdicated and his three sons divided the duchy. Two of the branches founded at this time soon died out; and in 1569, after the death of Ernest I., the representative of the third branch, his two sons agreed upon a partition which is of considerable importance in the history of Brunswick, since it established the lines of Dannenberg and of Lüneburg-Celle, and these two families divided the duchy of Brunswick-Wolfenbüttel in 1635. The dukes of Lüneburg-Celle subsequently took the name of Hanover, and were the ancestors of the later kings of Hanover (q.v.). After the acquisition of 1635 the family of Dannenberg took the title of Brunswick-Wolfenbüttel, and ruled in the direct line until 1735. It was then followed by the family of Brunswick-Bevern, which had split off from the parent line in 1666 and ruled until 1884.

Brunswick has not played a very important part in German politics. Many counties were added to its area, but it was weakened by constant divisions of territory, and during the period of the Reformation some of the princes took one side and some the other. The treaty of Westphalia in 1648 made little difference to its prestige, but its subsequent position was greatly affected by the growth of Prussia. During the Seven Years' War Brunswick supported Frederick the Great, and in return was severely ravaged by the French. Duke Charles I., who accumulated a large amount of debt, sought to discharge his liabilities by sending his soldiers as mercenaries to assist England during the American War of Independence. The succeeding duke, Charles William Ferdinand, brought order into the finances, led the Prussian troops against Napoleon, and died in 1806 from wounds received at the battle of Auerstadt. Napoleon then declared the ducal family deposed and included Brunswick in the kingdom of Westphalia. In 1813 it was restored to Duke Frederick William, who was killed in 1815 at the battle of Quatre Bras. His son, Charles II., while a minor, was under the regency of George, afterwards the English king George IV., who ruled the duchy through Ernest, Count Münster-Ledenburg (1766-1839), assisted by Justus von Schmidt-Phiseldeck (1769-1851). A new constitution was granted in 1820, but after Charles came of age in 1823 a period of disorder ensued.

The duke, who was very unpopular with his subjects, quarrelled with his relatives, and in 1830 a revolution drove him from the country. The government was undertaken by his brother William, and in 1831 Charles was declared incapable of ruling, and William was appointed as his successor. The ex-duke, who made a fine collection of diamonds, died childless at Geneva in August 1873. William's long reign witnessed many excellent and necessary reforms. A new constitution was granted in 1832, and in 1844 Brunswick joined the Prussian Zollverein. Trial by jury and freedom of the press were established, many religious disabilities were removed, and measures were taken towards the freedom of trade.

Brunswick took very little part in the war between Prussia and Austria in 1866, but her troops fought for Prussia during the Franco-German War of 1870-71. The duchy joined the German Confederation in 1815, the North German Confederation in 1866, and became a state of the German empire in 1871.

In 1866 the question of the succession to Brunswick became acute. Duke William was unmarried, and according to the existing conventions it would pass to George, king of Hanover, who had just been deprived of his kingdom by the king of Prussia. In 1879, however, the duke and the estates, with the active support of Prussia, concluded an arrangement for a temporary council of regency to take over the government on William's death. Moreover, if in this event the rightful heir was unable to take possession of the duchy, the council was empowered to appoint a regent. William died on the 18th of October 1884, and George's son, Ernest, duke of Cumberland, claimed Brunswick and promised to respect the German constitution. This claim was disregarded by the council of regency, and the Bundesrat declared that the accession of the duke of Cumberland would be inimical to the peace and security of the empire on account of his attitude towards Prussia. In the following year the council chose Albert, prince of Prussia, as regent, a step which brought Brunswick still more under the influence of her powerful neighbour.

Albert died in September 1906, and after some futile negotiations with the duke of Cumberland, the Brunswick diet chose Duke John Albert of Mecklenburg-Schwerin (b. 1857) as regent in May 1907.

See O. von Heinemann, Geschichte Braunschweigs und Hannovers (Gotha, 1882-1892); W. Havemann, Geschichte der Lande Braunschweig und Lüneburg (Göttingen, 1853-1857); H. Sudendorf, Urkundenbuch zur Geschichte der Herzöge von Braunschweig und Lüneburg und ihrer Lande (Hanover, 1859-1883); H. Guthe, Die Lande Braunschweig und Hannover (Hanover, 1890); J. Beste, Geschichte der braunschweigischen Landeskirche von der Reformation bis auf unsere Tage (Wolfenbüttel, 1889); A. Köcher, Geschichte von Hannover und Braunschweig 1648-1714 (Leipzig, 1884).