Reuss (German: Reuß [ˈʁɔʏs] ^ⓘ) was the name of several historical states located in present-day Thuringia, Germany. Several lordships of the Holy Roman Empire which arose after 1300 and became Imperial Counties from 1673 and Imperial Principalities in the late 18th century were ruled by the House of Reuss.

A varying number of these counties came into being by partition; they were partially merged and divided again. After the end of the empire in 1806, the principality of the elder line, as well as several of the younger, became sovereign member states of the German Confederation, with the younger ones merging into a unified principality by 1848. The two remaining territories became federal principalities of the German Empire in 1871, the Principality of Reuss Elder Line with the state capital of Greiz and the Principality of Reuss Younger Line with the state capital of Gera. Both states were ruled by the House of Reuss until the German Revolution of 1918–1919. Reuss had a non-contiguous area of 1,143 square kilometers (441 square miles) and 211,324 inhabitants in 1919. The head of each branch bore the German title Fürst (Prince, as head of a princely house) while their children and all other members of the house bore the title Prinz/Prinzessin (Prince/Princess, as agnate members of a princely house).

Since the end of the 12th century, all male members of the House of Reuss are named Heinrich (English: Henry), in honour of Henry VI, Holy Roman Emperor (1190–1197), to whom they owed the dominions of Weida and Gera. For the purpose of differentiation, they are given order numbers according to certain systems (see below, section Numbering of the Heinrichs), and in private life they are distinguished by nicknames.

Several different principalities of the House of Reuss which had previously existed had by the time of the formation of the German Confederation become part of the two remaining lines (the Elder and the Younger lines). Before then, they had been part first of the Holy Roman Empire, and then the Confederation of the Rhine.

The region including what would become the Principality of Reuss was inhabited in early medieval times by Slavic people who were converted to Christianity by the German Emperor Otto I (936–973). In church matters the region was under the Diocese of Zeitz (founded in 968), which became a suffragan of Magdeburg. On account of the frequent inroads of the Slavs, the residence of the Bishop of Zeitz was removed to Naumburg in 1028, after which the See was called Naumburg-Zeitz.

Upon its subjection to German authority, the whole province was allotted to the March of Zeitz. As early as the year 1000, however, Emperor Otto III permitted the entire part lying on the eastern boundary of Thuringia, a wooded area, sparsely populated by the West Slavic people of the Sorbs, to be cleared for farmland and settled by German settlers. Emperor Henry IV appointed Henry the Pious of Gleissberg (c. 1040−1120) imperial vogt, or bailiff (advocatus imperii) of this settlement area, under the rule of the imperial Quedlinburg Abbey. He was a son of Erkenbert I of Weida, the oldest known ancestor of the family, who is mentioned in 1122 in the entourage of Count Adalbert of Everstein at the consecration of St John's church in Plauen. The name of the area Heinrich controlled derives from his office: Vogtland (Terra advocatorum, Land of the Bailiff). This designation has remained to this day a geographical summary for a region of 3,467 km² (comparable roughly to the county of Essex) which is located in Saxony, Thuringia and, to a lesser extent, in northern Bavaria.

The position of vogt (bailiff) soon became hereditary. While the dominions of Heinrich von Gleissberg included the towns Gera and Weida, his grandson Henry II the Rich (d. before 1209) also acquired Plauen. When his three sons divided their inheritance, three independent areas emerged, ruled by the branches of the bailiffs of Weida-Ronneburg, Plauen-Gera and Greiz-Reichenbach. The bailiffs, initially unfree nobles (Ministerialis), quickly rose to the rank of lords. After the division, the official title Vogt was carried on by all branches and passed on like a hereditary imperial fiefdom. When the bailiffs negotiated a treaty with Henry III, Margrave of Meissen in 1254, they acted as equal partners. In 1329 Emperor Ludwig the Bavarian confirmed the bailiffs a rank equal to Princes of the Holy Roman Empire, albeit without the title itself, they continued to use the designation Vogt.

In the 12th and 13th centuries, the bailiffs of Weida gradually became independent of the Quedlinburg Abbey on the lands they administered. Their area included what is generally understood today as Vogtland, largely in the present-day federal state of Thuringia, with a small part in the Free State of Bavaria. Over time the dominions of the bailiffs extended beyond the Vogtland into the Western Ore Mountains, with areas extending into what is now the Czech Republic. This cross-border Vogtland, as it is still called today, and whose territories ruled by the vogts fluctuated over the centuries, covers 1338 square miles, which is roughly the size of Santa Clara County in California.