There were no major crusades started by Scots, but there was Scottish participation in many of the crusades, though less so than from regions like France, England, and the Low Countries. The last two in particular were real centres of crusading fervour.

Scottish crusading began with the First Crusade. Numerous chroniclers list the Scots as one of the many different nations that answered Pope Urban's call for holy war. One of the few traceable Scots of the First Crusade was Lagmann, one of the early Lords of the Isles (Scoto-Norse petty kings of the western isles of Scotland) who probably joined Robert of Normandy's contingent.

Scots also participated in the Second and Third Crusades, though no specific names are known for the former. The Third Crusade saw Scoto-Norman participants like Robert de Quincy. Alan the Steward, ancestor of the Stewart kings of Scotland, may have also joined the crusade, certainly some of his knights seem to. David, earl of Huntingdon and brother to King William the Lion of Scotland, has often been claimed to have participated, but there is little evidence to support this.

The thirteenth century crusades again saw some Scottish participation and at a greater level than before. David Rufus of Forfar joined the Fourth Crusade and the de Quincy family went to the East soon after. Crusading Scots of that period also included two Gaelic bards Muiredhach Albanach O Dalaigh and Gille Brigde Albanach. It was not just Lowlander Scots that crusaded. In the 1240s the first Scottish earl, Patrick earl of Dunbar, joined a crusade. However, he died of an illness in southern France before he could reach the East. Another noble, David of Strathbogie, earl of Atholl, died at Tunis in Louis IX's Eighth Crusade in 1270. Adam de Kilconquhar, earl of Carrick, also joined Louis' crusade, but survived it and followed Edward I of England's Ninth Crusade to Acre the following year. Robert Bruce, lord of Annandale and ancestor of King Robert the Bruce, also joined this expedition as did his son. In the 1200s several members of the Stewart and Balliol families also crusaded.

In the late thirteenth century there were two events that conspired to suppress Scottish crusading for a time. Firstly, the last remnant of the crusader kingdom of Jerusalem, the city of Acre in modern Israel, fell to the Muslims in 1291. With this, crusading in the eastern mediterranean changed from campaigns aimed at conquering or defending the Holy Land into defensive campaigns trying to halt the Turkish advance in Anatolia, Greece, and the Balkans. Second to this, from 1296 Scotland and England were at war for three decades. Prospective Scottish crusaders were firstly distracted by a prolonged war at home and those that still wished to go abroad were largely unable to do so. There was no guarantee that their families, lands, or position would be safe in their absence, in spite of the church protection that accompanied a crusade vow.

The two kingdoms remained at war for much of the 1300s. When peace did occasionally break out, both the knights of England and Scotland could be found fighting on crusade abroad (the same occurs with English and French crusading during the Hundred Years War). It is important to note at this point that the crusade was not something restricted solely to the Middle East. Crusades were also carried out in Spain, which was partly held by Muslims, the Baltic, which was inhabited by many pagan tribes, and eastern Europe, where the Turks were encroaching on the Byzantine Empire. Some crusades were even directed against Christian heretics or schismatics.

What could be considered a Scottish 'crusade', though a minor one, occurred in 1329, after the death of Robert the Bruce. On his deathbed Robert asked that his heart be taken to the Church of the Holy Sepulchre in Jerusalem. This task was given to James Douglas. Douglas set out for the East with around 25 knights and esquires, but stopped in Spain to fight with the king of Castille against the Moors (the Muslims of Spain). Douglas was killed in a skirmish there in 1330 and Bruce's heart was returned to Scotland.

With the loss of the Holy Land, Scottish knights began to crusade elsewhere. A particularly popular destination was the Baltic, where the Teutonic Knights were fighting pagan Lithuania. Walter Leslie and his brother Norman, as well as a company of fourteen others went to the Baltic in 1356. Other Scottish contingents went in 1362, 1378, 1391, 1400, and 1410. In 1391 one Scottish crusader, William Douglas of Nithsdale, was murdered by English crusaders at Konigsberg (modern Kaliningrad). Scots were also present at the Alexandrian Crusade in 1365 but did not participate in the other eastern fourteenth-century crusades like Nicopolis (1396).

In 1410 the Teutonic Knights were defeated by a combined Polish-Lithuanian army at the Battle of Tannenberg (or Grunwald, or Zalgiris, depending on who you ask). This saw the end of their crusading campaigns in the Baltic. After this date there were very few Scottish crusaders, though the same can be said for much of Europe. A few Scots still went to the Holy Land, but it's unclear whether these were peaceful pilgrimages or armed crusades. Scots also joined campaigns against the Turks in Greece but not in the numbers of past centuries.

Scotland was also involved in crusading through the military orders. These were monastic societies of soldier-monks that fought in the crusades, the most famous of which were the Knights Templar. Four of these orders had lands in Scotland from which they raised money to fight in the East and also recruited new members. The Knights Templar were based at Balantrodoch (now Temple), near Edinburgh, and at Maryculter near Aberdeen. The Knights Hospitaller were headquartered in Torphichen, west of Edinburgh. The Order of St Lazarus had an estate in Edinburgh (the city's burgh church, St Giles', used to be theirs). The last military order in Scotland, the Knights of St Thomas of Acre, are barely documented. All that is known about them is that they had a hospital in Ayrshire.

The Templars, Hospitallers, and Lazarites all probably arrived in Scotland in the reign of David I (1124-53). Their initial membership in Scotland appears to have been English (each order's Scottish lands were run from England), but after the Wars of Independence from 1296, Scottish names begin to appear instead. During the Wars all three orders seem to have sided with the English. William Wallace drove out the Hospitallers from their preceptory (an estate) of Torphichen and held his only parliament there, whilst the master of the Scottish Temple fought and died in Edward I's army at the Battle of Falkirk in 1298. The Templars across Europe were suppressed on charges of heresy in 1312 and their Scottish lands largely passed to the Hospitallers. The Lazarites were eventually expelled from Scotland sometime in the reign of David II (1329-71) for being loyal to superiors in England. Only the Hospitallers rise to any real prominence in Scotland. In the late fifteenth century, the master of the Scottish Hospitallers, William Knollis, served as Treasurer of Scotland and was given a range of diplomatic and administrative offices and responsibilities. He was made a lord of parliament and probably participated in the rebellion that led to the death of King James III in 1488 (Knollis appears as a member of the rebels suspiciously quickly after James dies at the Battle of Sauchieburn). The Order was eventually ended in Scotland when the last master, the Protestant James Sandilands, asked Mary, Queen of Scots, to be allowed to turn the Order's estates into a secular barony. His heir is still around today as Lord Torphichen and maintains some ties to the modern Anglican revivals of the Hospitallers.

The only academic book on Scottish crusading I know of is Alan Macquarrie's Scotland and the Crusades (Edinburgh, 1985). Here you can find his two volume PhD thesis that he based the book on. There's also a recent work on the crusades and the whole of the British Isles, which includes some Scottish crusading: Kathryn Hurlock, Britain, Ireland and the Crusades, c.1000-1300 (Basingstoke, 2012). However, its author takes a Traditionalist approach to the crusades (meaning that she only counts crusades to the Holy Land), and so it misses out on the fourteenth century, when Scottish crusading in the Baltic was at its height. For the military orders in Scotland, see Macquarrie's PhD thesis linked above, or Ian B. Cowan, P. H. R. Mackay, and Alan Macquarrie (eds), The Knights of St John of Jerusalem in Scotland (Edinburgh, 1983). Despite the title, it covers the Scottish Templars as well.