The Crosiers, formally known as the Canons Regular of the Order of the Holy Cross (Latin: Canonici Regulares Ordinis Sanctae Crucis), abbreviated OSC, is a Catholic religious order of canons regular of Pontifical Right for men. It is one of the Church's oldest religious orders, and membership consists of priests and brothers who live together according to the Rule of St. Augustine.

The Crosiers were founded by five men attached to the household of the prince-bishop of Liege, Rudolf of Zähringen, who accompanied the Emperor Frederick Barbarossa on the Third Crusade (1189–1191). Upon their return, the five, led by Theodorus de Cellis (1166–1236), sought a new way of life, and shortly before his death, their bishop appointed them to be canons of his St. Lambert's Cathedral, Liège.

After efforts to renew the life and practice of the college of canons to which they belonged, the five withdrew from Liège and moved up the Meuse River to a place called Clairlieu, outside the city of Huy, and began a way of life more in keeping with their ideals. This settlement of the five at Huy was the beginning of their Order, and the house and small church dedicated to Saint Theobald that they established there became the Order's motherhouse. Pope Innocent III verbally approved their Order on the feast day of the Finding of the Holy Cross, 3 May 1210, and Pope Innocent IV granted them full and final approval on 3 May 1248 the Feast of the Finding of the Holy Cross.

In 1410, the Crosiers' general chapter ordered the destruction of its records and decisions from the time of its foundation. The reason for this radical act is recorded to have been a thorough reformation of some sort, but it left the Order's modern historians with only fragments and clues to their Order's first two centuries, and the tradition summarized above.

The principal source of information about the origin of the order is in the Chronicon Cruciferorum of Henricus Russelius, Prior of Suxy. Their own sources, and mention of them in non-Crosier sources, usually call them "the Brethren of the Holy Cross," and the French and English words used for them, Croisiers and Crosiers, are derived from the French "croisé", one of the words used for a crusader, and meaning "marked with a cross."

Only one of their five founders for whom they have a name is the group's leader, and that only in its Latin form, Theodoricus (or Diederick) de Cellis, which first appears in a short history of the Order published in 1636. While Rusellius does not mention Theodore's parents, there are biographies from the 17th century that say he was the son of Walter de Beaufort and Oda de Celles, guardians of the abbatial church of Celles near Dinant during the latter half of the 12th century.

There is no record of the presence of the Crosiers at Huy until the 1240s, and only in 1322 did Clairlieu become the site of a magnificent church dedicated to the Holy Cross instead of the small chapel of St. Theobald.

The new institution soon extended to France, the Netherlands, Germany, and also to England. Because they were established in the early 13th century, they were contemporaries of the Dominicans and Franciscans, they were frequently misidentified as friars and were often confused with other religious orders known as Crosiers who identified themselves with the Holy Cross. So, for example, there was a very old tradition that Bishop Albert of Prague took several Crosiers with him to Livonia, but these were in fact members of the Bohemian order of the Holy Cross. In England, too, they and an Italian order of the Holy Cross were both identified as Crutched Friars, and so the location of their houses and their activities are often mistaken for each other.^{[citation needed]}