William I, Prince of Orange (24 April 1533 – 10 July 1584), also widely known as William the Silent (Dutch: Willem de Zwijger), or simply William of Orange (Dutch: Willem van Oranje), was the main leader of the Dutch revolt against the Spanish that set off the Eighty Years' War and resulted in the formal independence of the United Provinces in 1648. He was born into the House of Nassau as a count of Nassau-Dillenburg. He became Prince of Orange in 1544 and is thereby the founder of the branch House of Orange-Nassau.

A wealthy nobleman, William originally served the Habsburgs as a member of the court of Margaret of Parma, governor of the Spanish Netherlands. Unhappy with the centralisation of political power away from the local estates and with the Spanish persecution of Dutch Protestants, William joined the Dutch uprising and turned against his former masters. The most influential and politically capable of the rebels, he led the Dutch to several successes in the fight against the Spanish. Declared an outlaw by the Spanish king in 1580, he was assassinated by Balthasar Gérard (also written as 'Gerardts') in Delft four years later.

William was born on 24 April 1533 in the castle of Dillenburg in Nassau, Germany. He was the eldest son of William, Count of Nassau and Juliana of Stolberg-Werningerode, and was raised a Lutheran. He had four younger brothers and seven younger sisters: John, Hermanna, Louis, Mary, Anna, Elisabeth, Katharine, Juliane, Magdalene, Adolf and Henry.

When his cousin, René of Châlon, Prince of Orange, died childless in 1544, the eleven year old William inherited all Châlon's property, including the title Prince of Orange, on the condition that he receive a Roman Catholic education. Besides Châlon's properties, he also inherited vast estates in the Low Countries (present-day Netherlands and Belgium). Because of his young age, Holy Roman Emperor Charles V served as the regent of the principality until William was fit to rule. William was sent to the Netherlands to receive the required education, first at the family's estate in Breda, later in Brussels under the supervision of Mary of Habsburg (Mary of Hungary), the sister of Charles V and governor of the Habsburg Netherlands (Seventeen Provinces). In Brussels, he was taught foreign languages and received military and diplomatic education under the direction of Champagney (Jérôme Perrenot), brother of Granvelle.

On 6 July 1551, he married Anna van Egmond en Buren, the wealthy heir to the lands of her father, and William gained the titles Lord of Egmond and Count of Buren. They had three children. Later that same year, William was appointed captain in the cavalry. Favoured by Charles V, he was rapidly promoted, and became commander of one of the Emperor's armies at age 22. He was made a member of the Raad van State, the highest political advisory council in the Netherlands in November 1555, shortly after Charles had abdicated in favour of his son, Philip II of Spain. It was on the shoulder of William that the gout-afflicted Emperor leaned during his abdication ceremony.

His wife Anna died on 24 March 1558. Later, William had a brief relationship with Eva Elincx, leading to the birth of their illegitimate son, Justinus van Nassau: William officially recognised him and took responsibility for his education — Justinus would become an admiral in his later years.

In 1559, Philip appointed William as the stadtholder (governor) of the provinces Holland, Zeeland and Utrecht, thereby greatly increasing his political power. A stadtholdership over Franche-Comté followed in 1561.

Although he never directly opposed the Spanish king, William soon became one of the most prominent members of the opposition in the Council of State, together with Philip de Montmorency, Count of Hoorn and Lamoral, Count of Egmont. They were mainly seeking more political power, for themselves against the de facto government of Count Berlaymont, Granvelle and Viglius of Aytta, but also for the Dutch nobility and, ostensibly, for the Estates, and complained that too many Spaniards were involved in governing the Netherlands. William was also dissatisfied with the increasing persecution of Protestants in the Netherlands. Brought up as both a Lutheran and later a Catholic, William was very religious but still was a proponent of freedom of religion for all people. The inquisition policy in the Netherlands, carried out by Cardinal Granvelle, prime minister to the new governor Margaret of Parma (1522 – 83) (natural half-sister to Philip II), increased opposition to the Spanish rule among the — then mostly Catholic — population of the Netherlands. Lastly, the members of the opposition wished an end to the presence of Spanish troops.

On 25 August 1561, William of Orange married for the second time. His new wife, Anna of Saxony, was described by contemporaries as "self-absorbed, weak, assertive, and cruel", and it is generally assumed that William married her to gain more influence in Saxony, Hesse and the Palatine. The couple had five children.

Up to 1564, any criticism of governmental measures voiced by William and the other members of the opposition had ostensibly been directed at Granvelle; however, after the latter's depart early that year, William, who may have found increasing confidence in his alliance with the Protestant princes of Germany following his second marriage, began to openly criticize the King's anti-Protestant politics. In an iconic speech to the Council of State, William to the shock of his audience motivated his conflict with king Philip II by saying that, even though he had decided for himself to keep to the Catholic faith, he could not approve that monarchs should desire to rule over the souls of their subjects and take away from them their freedom of belief and religion.

Later, in his Apology (1580), William stated that his resolve to oppose the King's policies had originated in June 1559, when, during a hunting trip to the Bois de Vincennes together with the duke of Alva and king Henry II of France, to whom both had been sent as hostages to ensure the proper fulfillment of the conditions of the Treaty of Cateau-Cambrésis following the Hispano-French war, the latter two had openly discussed a secret understanding between Philip and Henry which aimed at the extermination of the Protestants in both France and the Netherlands; William at that time had kept his silence, but had decided for himself that he would not allow the slaughter of so many innocent subjects.

In early 1565, a large group of lesser noblemen, including William's younger brother Louis, formed the Confederacy of Noblemen. On 5 April, they offered a petition to Margaret of Parma, requesting an end to the persecution of Protestants. From August to October 1566, a wave of iconoclasm (known as the Beeldenstorm) spread through the Low Countries. Calvinists, Anabaptists and Mennonites, angry with their being persecuted by the Roman Catholic Church and opposed to the Catholic images of saints (which in their eyes conflicted with the Second Commandment), destroyed statues in hundreds of churches and monasteries throughout the Netherlands.

Following the Beeldenstorm, unrest in the Netherlands grew, and Margaret agreed to grant the wishes of the Confederacy, provided the noblemen would help to restore order. She also allowed more important noblemen, including William of Orange, to assist the Confederacy. In late 1566, and early 1567, it became clear that she would not be allowed to fulfill her promises, and when several minor rebellions failed, many Calvinists (the major Protestant denomination) and Lutherans fled the country. Following the announcement that Philip II, unhappy with the situation in the Netherlands, would dispatch his loyal general Fernando Álvarez de Toledo, Duke of Alba (also known as "The Iron Duke") to restore order, William laid down his functions and retreated to his native Nassau in April 1567. He had been (financially) involved with several of the rebellions.

After his arrival in August 1567, Alba established the Council of Troubles (known to the people as the Council of Blood) to judge those involved with the rebellion and the iconoclasm. William was one of the 10,000 to be summoned before the Council, but he failed to appear. He was subsequently declared an outlaw, and his properties were confiscated. As one of the most prominent and popular politicians of the Netherlands, William of Orange emerged as the leader of an armed resistance. He financed the Watergeuzen, refugee Protestants who formed bands of corsairs and raided the coastal cities of the Netherlands (often killing Spanish and Dutch alike). He also raised an army, consisting mostly of German mercenaries to fight Alba on land. William allied with the French Huguenots, following the end the second Religious War in France when they had troops to spare. Led by his brother Louis, the army invaded the northern Netherlands in 1568. However the plan failed almost from the start. The Huguenots were defeated by French Royal Troops before they could invade, and a small force under Jean de Villers was captured within two days. Villers gave all the plans of the campaign to the Spanish following this capture. On 23 May, the army under the command of Louis won the Battle of Heiligerlee in the northern province of Groningen against a Spanish army led by the stadtholder of the northern provinces, Jean de Ligne, Duke of Aremberg. Aremberg was killed in the battle, as was William's brother Adolf. Alba countered by killing a number of convicted noblemen (including the Counts of Egmont and Hoorn on 6 June), and then by leading an expedition to Groningen. There, he annihilated Louis’ forces on German territory in the Battle of Jemmingen on 21 July, although Louis managed to escape. These two battles are now considered to be the start of the Eighty Years' War.

William responded by leading a large army into Brabant, but Alba carefully avoided a decisive confrontation, expecting the army to fall apart quickly. As William advanced, riots broke out in his army, and with winter approaching and money running out, William decided to turn back. William made several more plans to invade in the next few years, but little came of it, lacking support and money. He remained popular with the public, partially through an extensive propaganda campaign through pamphlets. One of his most important claims, with which he attempted to justify his actions, was that he was not fighting the rightful owner of the land, the Spanish king, but only the inadequate rule of the foreign governors in the Netherlands, and the presence of foreign soldiers. On 1 April 1572 a band of Watergeuzen captured the city of Brielle, which had been left unattended by the Spanish garrison. Contrary to their normal "hit and run" tactics, they occupied the town and claimed it for the prince by raising the Prince of Orange's flag above the city. This event was followed by other cities in opening their gates for the Watergeuzen, and soon most cities in Holland and Zeeland were in the hands of the rebels, notable exceptions being Amsterdam and Middelburg. The rebel cities then called a meeting of the Staten Generaal (which they were technically unqualified to do), and reinstated William as the stadtholder of Holland and Zeeland.

Concurrently, rebel armies captured cities throughout the entire country, from Deventer to Mons. William himself then advanced with his own army and marched into several cities in the south, including Roermond and Leuven. William had counted on intervention from the French Protestants (Huguenots) as well, but this plan was thwarted after the St. Bartholomew's Day Massacre on 24 August, which signalled the start of a wave of violence against the Huguenots. After a successful Spanish attack on his army, William had to flee and he retreated to Enkhuizen, in Holland. The Spanish then organised countermeasures, and sacked several rebel cities, sometimes massacring their inhabitants, such as in Mechelen or Zutphen. They had more trouble with the cities in Holland, where they took Haarlem after seven months and a loss of 8,000 soldiers, and they had to give up their siege of Alkmaar.

In 1573, William went over to the Calvinist Church.

In 1574, William's armies won several minor battles, including several naval encounters. The Spanish, lead by Don Luis de Zúñiga y Requesens since Philip replaced Alba in 1573, also had their successes. Their decisive victory in the Battle of Mookerheyde in the south east, on the Meuse embankment, on 14 April cost the lives of two of William's brothers, Louis and Henry. Requesens's armies also besieged the city of Leiden. They broke up their siege when nearby dykes were cut by the Dutch. William was very content with the victory, and established the University of Leiden, the first university in the Northern Provinces.

William had his previous marriage legally disbanded in 1571, on claims that his wife Anna was insane. He then married for the third time on 24 April 1575 to Charlotte de Bourbon-Monpensier, a former French nun, who was also popular with the public. Together, they had six daughters.

After failed peace negotiations in Breda in 1575, the war lingered on. The situation improved for the rebels when Don Requesens died unexpectedly in March 1576, and a large group of Spanish soldiers, not having received their salary in months, mutinied in November of that year and unleashed the Spanish Fury on the city of Antwerp, a tremendous propaganda coup for the Dutch Revolt. While the new governor, Don John of Austria, was under way, William of Orange managed to have most of the provinces and cities sign the Pacification of Ghent, in which they declared to fight for the expulsion of Spanish troops together. However, he failed to achieve unity in matters of religion. Catholic cities and provinces would not allow freedom for Calvinists, and vice versa.

When Don John signed the Perpetual Edict in February 1577, promising to comply with the conditions of the Pacification of Ghent, it seemed that the war had been decided in favour of the rebels. However, after Don John took the city of Namur in 1577, the uprising spread throughout the entire Netherlands. Don John attempted to negotiate peace, but the prince intentionally let the negotiations fail. On 24 September 1577, he made his triumphal entry in the capital Brussels. At the same time, Calvinist rebels grew more radical, and attempted to forbid Catholicism in their areas of control. William was opposed to this both for personal and political reasons. He desired freedom of religion, and he also needed the support of the less radical Protestants and Catholics to reach his political goals. On 6 January 1579, several southern provinces, unhappy with William's radical following, sealed the Treaty of Arras, in which they agreed to accept their governor, Alessandro Farnese, Duke of Parma (who had succeeded Don John). Five northern provinces, later followed by most cities in Brabant and Flanders, then signed the Union of Utrecht on 23 January, confirming their unity. William was initially opposed to the Union, as he still hoped to unite all provinces. Nevertheless, he formally gave his support on 3 May. The Union of Utrecht would later become a de facto constitution, and would remain the only formal connection between the Dutch provinces until 1795.

In spite of the renewed union, the Duke of Parma was successful in reconquering most of the southern part of the Netherlands. Because he had agreed to remove the Spanish troops from the provinces under the Treaty of Arras, and because Philip II needed them elsewhere subsequently, the Duke of Parma was unable to advance any further until the end of 1581. In the mean time, William and his supporters were looking for foreign support. The prince had already sought French assistance on several occasions, and this time he managed to gain the support of François, Duke of Anjou, brother of king Henry III of France. On September 29, 1580, the Staten Generaal (with the exception of Zeeland and Holland) signed the Treaty of Plessis-les-Tours with the Duke of Anjou. The Duke would gain the title "Protector of the Liberty of the Netherlands" and become the new sovereign. This, however, required that the Staten Generaal and William would let go of their formal support of the King of Spain, which they had maintained officially up to that moment.

On 22 July 1581, the Staten Generaal declared their decision to no longer recognise Philip II as their king, in the Act of Abjuration. This formal declaration of independence enabled the Duke of Anjou to come to the aid of the resisters. He did not arrive until 10 February 1582, when he was officially welcomed by William in Flushing. On 18 March, the Spaniard Juan de Jáuregui attempted to assassinate William in Antwerp. Although William suffered severe injuries, he survived thanks to the care of his wife Charlotte and his sister Mary. While William slowly recovered, the intensive care by Charlotte took its toll, and she died on 5 May. The Duke of Anjou was not very popular with the population. The provinces of Zeeland and Holland refused to recognise him as their sovereign, and William was widely criticised for what were called his "French politics". When the Anjou's French troops arrived in late 1582, William's plan seemed to pay off, as even the Duke of Parma feared that the Dutch would now gain the upper hand.

However, the Duke of Anjou himself was displeased with his limited power, and decided to take the city of Antwerp by force on 18 January 1583. The citizens, who were warned in time, defended their city in what is known as the "French Fury". Anjou's entire army was killed, and he received reprimands from both Catherine de Medici and Elizabeth I of England (who he had courted). The position of Anjou after this attack became impossible to hold, and he eventually left the country in June. His leave also discredited William, who nevertheless maintained his support for Anjou. He stood virtually alone on this issue, and became politically isolated. Holland and Zeeland nevertheless maintained him as their stadtholder, and attempted to declare him count of Holland and Zeeland, thus making him the official sovereign. In the middle of all this, William had married for the fourth and final time on 12 April 1583 to Louise de Coligny, a French Huguenot and daughter of Gaspard de Coligny. She would be the mother of Frederick Henry (1584 – 1647), William's fourth legitimate son.

The Catholic Frenchman Balthasar Gérard (born 1557) was a supporter of Philip II, and in his opinion, William of Orange had betrayed the Spanish king and the Catholic religion. After Philip II declared William an outlaw and promised a reward of 25,000 crowns for his assassination, and of which Gérard learned in 1581, he decided to travel to the Netherlands to kill William. He served in the army of the governor of Luxembourg, Peter Ernst I von Mansfeld-Vorderort for two years, hoping to get close to William when the armies met. This never happened, and Gérard left the army in 1584. He went to the Duke of Parma to present his plans, but the Duke was unimpressed. In May 1584, he presented himself to William as a French nobleman, and gave him the seal of the Count of Mansfelt. This seal would allow for forgeries of messages of Mansfelt. William sent Gérard back to France to pass the seal to his French allies. Gérard returned in July, having bought pistols on his return voyage. On 10 July, he made an appointment with William of Orange in his home in Delft, nowadays known as the Prinsenhof. That day, William was having dinner with his guest Rombertus van Uylenburgh. After William left the dining room and climbed down the stairs, Van Uylenburgh heard how Gérard shot William in the chest from close range. Gérard fled to collect his reward. According to official records, William's last words are said to have been:

("My God, have pity on my soul; my God, have pity on this poor people.")

Gérard was caught before he could flee Delft, and imprisoned. He was tortured before his trial on 13 July, where he was sentenced to be brutally — even by the standards of that time — killed. The magistrates sentenced that the right hand of Gérard should be burned off with a red-hot iron, that his flesh should be torn from his bones with pincers in six different places, that he should be quartered and disemboweled alive, that his heart should be torn from his bosom and flung in his face, and that, finally, his head should be cut off.

Traditionally, members of the Nassau family were buried in Breda, but as that city was in Spanish hands when William died, he was buried in the New Church in Delft. His grave monument was originally very sober, but it was replaced in 1623 by a new one, made by Hendrik de Keyser and his son Pieter. Since then, most of the members of the House of Orange-Nassau, including all Dutch monarchs have been buried in the same church. His great-grandson William the third, King of England and Scotland and Stadtholder in the Netherlands was buried in Westminster Abbey

According to British historian of science Lisa Jardine, he is reputed to be the first world head of state assassinated through use of a handgun, though this is debatable since William was not officially head of state, and the Scottish Regent Moray was shot 13 years earlier.

Philip William, William's eldest son from his first marriage, to Anna of Egmond, succeeded him as Prince of Orange at the suggestion of Johan van Oldenbarneveldt. Phillip William died in Brussels on 20 February 1618 and was succeeded by his half-brother Maurice, the eldest son from William's second marriage, to Anna of Saxony, who became Prince of Orange. A strong military leader, he won several victories over the Spanish. Van Oldenbarneveldt managed to sign a very favourable twelve-year armistice in 1609, although Maurice was unhappy with this. Maurice was a heavy drinker and died on 23 April 1625 from liver disease. Maurice had several sons with Margaretha van Mechelen, but he never married her. So, Frederick Henry, Maurice's half-brother (and William's youngest son from his fourth marriage, to Louise de Coligny) inherited the title of Prince of Orange. Frederick Henry continued the battle against the Spanish. Frederick Henry died on 14 March 1647 and is buried with his father William "The Silent" in Nieuwe Kerk, Delft. The Netherlands became formally independent after the Peace of Westphalia in 1648.

The son of Frederick Henry, William II of Orange succeeded his father as stadtholder, as did his son, William III of Orange. The latter also became king of England, Scotland and Ireland from 1689. Although he was married to Mary II, Queen of Scotland and England for 17 years, he died childless in 1702. He appointed his cousin Johan Willem Friso (William's great-great-great-grandson) as his successor. Because Albertine Agnes, a daughter of Frederick Henry, married William Frederik of Nassau-Dietz, the present royal house of the Netherlands descends from William the Silent through the female line. As the chief financer and political and military leader of the early years of the Dutch revolt, William is considered a national hero in the Netherlands, even though he was born in Germany, and usually spoke French. Many of the Dutch national symbols can be traced back to William of Orange:

• The flag of the Netherlands (red, white and blue) is derived from the flag of the prince, which was orange, white and blue.
• The coat of arms of the Netherlands is based on that of William of Orange. Its motto Je maintiendrai (French, "I will maintain") was also used by William of Orange, who based it on the motto of his cousin René of Châlon, who used Je maintiendrai Châlon.
• The national anthem of the Netherlands, Het Wilhelmus, was originally a propaganda song for William. It was probably written by Philips van Marnix, lord of Sint-Aldegonde, a supporter of William of Orange.
• The national colour of the Netherlands is orange, and it is used, among other things, in clothing of Dutch athletes.
• The orange sash of the Prussian Order of the Black Eagle was in honour of the Dutch Dynasty of William the Silent, since the order's founder, Frederick I of Prussia's mother, Louise Henrietta of Nassau, was the granddaughter of William the Silent.
• A statue of William the Silent stands at the main campus of Rutgers University in New Brunswick, New Jersey, a legacy of the university's founding by ministers of the Dutch Reformed Church in 1766. The statue is commonly known to students and alumni as "Willie the Silent" and contains an inscription referring to William as "Father of his Fatherland."
• In January 2008, a planetoid was named after him.