William of Rubruck was a Flemish Fransican missionary and explorer. His account is one of the masterpieces of medieval geographical literature comparable to that of Marco Polo. Known also as William of Rubruk, Willem van Ruysbroeck, Guillaume de Rubrouck or Willielmus de Rubruquis. William accompanied Louis IX on the Seventh Crusade in 1248. On May 7, 1253, on Louis' orders, he set out from Constantinople a missionary journey to convert the Tartars. He actually followed the route of first journey of the Hungarian Friar Jullian. With William's party were Bartolomeo da Cremona, an attendant called Gosset, and an interpreter named Homo Dei, "man of God", literally translating Abdullah.

After reaching the Crimean town of Sudak, William continued his trek with oxen and carts. Nine days after crossing the Don he met Sartaq Khan, ruler of the Kipchak Khanate. The Khan sent William on to his father, Batu Khan, at Sarai near the Volga. Five weeks later, after the departure from Sudak, he reached the encampment of Batu Khan, Mongol ruler of the Volga River region. Batu refused conversion but sent the ambassadors on to the great Mongol Mangu Khan. He and his travelling companions set off on horseback on September 16, 1253, on a 5000 miles journey to the court of the Great Khan at Karakourum. Upon arrival they were received courteously and stayed at the Khan's camp until July 10, 1254, when they began their long journey back home. William and his companions reached the Crusader State of Tripoli on August 15, 1253. William of Rubruck's was the fourth European mission to the Mongols. Before William of Rubruck went Giovanni da Pian del Carpine and Ascelin of Lombardia in 1245 and Andre de Longiumeau in 1249The King was encouraged to send another mission by reports of the presence of Nestorian Christians at the Mongolian court.

On his return, William presented to the king a very clear and precise report, entitled

Itinerarium fratris Willielmi de Rubruquis de ordine fratrum Minorum, Galli, Anno gratia 1253 ad partes Orientales.

In this report, he described the peculiarities of Mongolia as well as many geographical observations, making it the first scientific description of central Asia. There were also anthropological observations, such as his surprise at the presence of Islam in Inner Asia.

William also answered a long-standing question, demonstrating by his passage north of the Caspian, that it was an inland sea and did not flow into the Arctic Ocean; although earlier Scandinavian explorers had doubtless already known this, he was the first to report it.

William's report is divided into 40 chapters. Chapters 1 - 10 relate general observations about the Mongols and their customs. Chapters 11 - 40 give an account of the course and the events of William's voyage.

The report of William of Rubruck is one of the great masterpieces of medieval geographical literature, comparable to that of Marco Polo, although they are very different. William was a good observer, and an excellent writer. He asked many questions along the way and did not take folk tale and fable as truth.

At one point of his stay among the Mongols, William entered into a famous competition at the Mongol court, as the khan encouraged a formal debate between the Christians, Buddhists, and Muslims, to determine which faith was correct, as determined by three judges, one from each faith. The debate drew a large crowd, and as with most Mongol events, a great deal of alcohol was involved. As described by Jack Weatherford in his book Genghis Khan and the Making of the Modern World:

Carpini account was purely factual but Rubruck wrote a more lively account. In A Journey to the Eastern Parts of the World, everything he wrote about his personal experiences has the ring of truth. He provided a detailed account of Mongol life and customs.