Henry IV (German: Heinrich IV; 11 November 1050 – 7 August 1106) was Holy Roman Emperor from 1084 to 1105, King of Germany from 1054 to 1105, King of Italy and Burgundy from 1056 to 1105, and Duke of Bavaria from 1052 to 1054. He was the son of Henry III, Holy Roman Emperor—the second monarch of the Salian dynasty—and Agnes of Poitou. After his father's death on 5 October 1056, Henry was placed under his mother's guardianship. She made grants to German aristocrats to secure their support. Unlike her late husband, she could not control the election of the popes, thus the idea of the "liberty of the Church" strengthened during her rule. Taking advantage of her weakness, Archbishop Anno II of Cologne kidnapped Henry in April 1062. He administered Germany until Henry came of age in 1065.

Henry endeavoured to recover the royal estates that had been lost during his minority. He employed low-ranking officials to carry out his new policies, causing discontent in Saxony and Thuringia. Henry crushed a riot in Saxony in 1069 and overcame the rebellion of the Saxon aristocrat Otto of Nordheim in 1071. The appointment of commoners to high office offended German aristocrats, and many of them withdrew from Henry's court. He insisted on his royal prerogative to appoint bishops and abbots, although the reformist clerics condemned this practice as simony (a forbidden sale of church offices). Pope Alexander II blamed Henry's advisors for his acts and excommunicated them in early 1073. Henry's conflicts with the Holy See and the German dukes weakened his position and the Saxons rose up in open rebellion in the summer of 1074. Taking advantage of a quarrel between the Saxon aristocrats and peasantry, he forced the rebels into submission in October 1075.

Henry adopted an active policy in Italy, alarming Pope Alexander II's successor, Gregory VII, who threatened him with excommunication for simony. Henry persuaded most of the German bishops to declare the Pope's election invalid on 24 January 1076. In response, the Pope excommunicated Henry and released his subjects from their allegiance. German aristocrats who were hostile to Henry called for the Pope to hold an assembly in Germany to hear Henry's case. To prevent the Pope from sitting in judgement on him, Henry went to Italy as far as Canossa to meet with the Pope. His penitential "Walk to Canossa" was a success and Gregory VII had no choice but to absolve him in January 1077. Henry's German opponents ignored his absolution and elected an antiking, Rudolf of Rheinfelden, on 14 March 1077. The Pope was initially neutral in the two kings' conflict, enabling Henry to consolidate his position. Henry continued to appoint high-ranking clerics, for which the Pope again excommunicated him on 7 March 1080. Most German and northern Italian bishops remained loyal to Henry and they elected the antipope Clement III. Rudolf of Rheinfelden was killed in battle and his successor, Hermann of Salm, could only exert royal authority in Saxony. From 1081, Henry launched a series of military campaigns to Italy, and Clement III crowned him emperor in Rome on 1 April 1084.

Hermann of Salm died and Henry pacified Saxony with the local aristocrats' assistance in 1088. He launched an invasion against the pope's principal Italian ally, Matilda of Tuscany, in 1089. She convinced Henry's elder son, Conrad II, to take up arms against his father in 1093. Her alliance with Welf I, Duke of Bavaria, prevented Henry's return to Germany until 1096 when he was reconciled with Welf. After Clement III's death, Henry did not support new antipopes, but did not make peace with Pope Paschal II. Henry proclaimed the first Reichsfriede (imperial peace) which covered the whole territory of Germany in 1103. His younger son, Henry V, forced him to abdicate on 31 December 1105. He tried to regain his throne with the assistance of Lotharingian aristocrats, but became ill and died without receiving absolution from his excommunication. Henry's preeminent role in the Investiture Controversy, his "Walk to Canossa" and his conflicts with his sons and wives established his controversial reputation, with some regarding him as the stereotype of a tyrant, and others describing him as an exemplary monarch who protected the poor.

Henry was the third monarch of the Salian dynasty—the royal house ruling Germany from 1024 to 1125. The 11th-century kings of Germany also ruled Italy and Burgundy and had a strong claim to the title of Holy Roman Emperor. They were convinced that their claim to the emperorship entitled them each to act as the head of all Christians and to control papal elections in Rome. Rome was actually dominated by local aristocrats, the Tusculani and the Crescentii, who raised their own candidates to the papal throne. Their rivalries caused scandals, culminating in three rival popes—Benedict IX, Sylvester III and Gregory VI—in 1045. To put an end to the schism, Henry's father, Henry III, crossed the Alps to Italy and held a church synod at Sutri on 20 December 1046. The synod deposed the three popes and replaced them with a German prelate, Bishop Suidger of Bamberg, who assumed the name Clement II.

Henry III emphasized the priestly nature of kingship, attributing it to the kings' anointment by holy oil. A man of great personal piety, he regarded himself as "Vicar of Christ", authorized to administer state and church alike. The Romans awarded him the hereditary title of patrician, acknowledging his and his successors' right to cast the first vote at papal elections. His new title enabled him to secure the appointment of German clerics to the papal throne. The third German pope, Leo IX, came from Lotharingia—a province that had been an important centre of reformist clerics. They wanted to purify the Church through the re-implementation of ancient (or supposedly ancient) collections of canon law and Leo IX enthusiastically introduced their ideas to Rome. He prohibited simony—the sale of church offices—and promoted clerical celibacy. Imperial control of church affairs was in the long run incompatible with the reformist idea of "liberty of the Church" which claimed that ecclesiastic institutions could only be subject to the authority of the Holy See. The conflict between the two ideas reached its pinnacle during Henry IV's reign, developing into the confrontation known as the Investiture Controversy.

Germany, Italy and Burgundy were composed of semi-independent provinces, each administered by a prelate or a lay aristocrat. The prelates—the bishops and abbots—were not only wealthy landowners, they also played an important role in state administration. They were required to make annual gifts to the kings and also to provide the monarchs with well-defined regular services, including the collection of taxes and hospitality. The dukes were the most powerful lay aristocrats in Germany. They were primarily military commanders, but they were also responsible for the administration of justice. The monarchs occasionally kept the office of duke for themselves or for their closest relatives, but sooner or later they had no choice but to fill vacant duchies, because they depended on the most powerful aristocrats' support.

Henry III came into conflict with influential dukes towards the end of his life. Godfrey the Bearded, Duke of Upper Lotharingia, married a wealthy widow, Margravine Beatrice of Tuscany, without the Emperor's consent. Henry III also outraged the Saxon duke, Bernard II, because he supported the duke's main rival, Archbishop Adalbert of Hamburg, in the seizure of some Saxon counties. The Salian kings who inherited their Ottonian predecessors' domains in Saxony visited the province frequently. Their lengthy visits irritated the Saxon aristocrats who were exposed to direct royal control for more time than their peers in other parts of the empire. The Saxons' grievances against the Salian monarchs broke out in a series of revolts during Henry IV's reign.