Mercenary
The barbarians were Romanized and surviving veterans were established in areas requiring population. The Varangian Guard of the Byzantine Empire is the best known formation made up of barbarian mercenaries (see next section).

Other: Members of independent Thracian tribes such as the Bessi and Dii often joined the ranks of large organized armies as mercenaries.
The Sons of Mars were Italian mercenaries used by the Greek kings of Syracuse until after the Punic Wars.
A figure in oral legend, Milesius was given the princess Scota after conducting a successful campaign for Ancient Egypt.
Mithridates VI Eupator recruited a large number of Iranians along with the Galatians into the Pontic army during the Mithridatic Wars against Rome, using the Leucosyri, Persians and Scythians.
Illyrians were hired across the Balkans and further. They were known for their unreliability.

Medieval warfare: Byzantine emperors followed the Roman practice and contracted foreigners especially for their personal corps guard called the Varangian Guard. They were chosen among war-prone peoples, of whom the Varangians (Norsemen) were preferred. Their mission was to protect the Emperor and Empire and since they did not have links to the Greeks, they were expected to be ready to suppress rebellions. One of the most famous guards was the future king Harald III of Norway, also known as Harald Hardrada ("Hard-counsel"), who arrived in Constantinople in 1035 and was employed as a Varangian Guard. He participated in eighteen battles and was promoted to akolythos, the commander of the Guard, before returning home in 1043. He was killed at the Battle of Stamford Bridge in 1066 when his army was defeated by an English army commanded by King Harold Godwinson. The point at which the Varangians ceased to be in the service of the Roman Empire remains unclear.
In England at the time of the Norman Conquest, Flemings (natives of Flanders) formed a substantial mercenary element in the forces of William the Conqueror with many remaining in England as settlers under the Normans. Contingents of mercenary Flemish soldiers were to form significant forces in England throughout the time of the Norman and early Plantagenet dynasties (11th and 12th centuries). A prominent example of these were the Flemings who fought during the English civil wars, known as the Anarchy or the Nineteen-Year Winter (AD 1135 to 1154), under the command of William of Ypres, who was King Stephen's chief lieutenant from 1139 to 1154 and who was made Earl of Kent by Stephen.
In Italy, the condottiero was a military chief offering his troops, the condottieri, to Italian city-states. The condottieri were extensively used by the Italian city-states in their wars against one another. At times, the condottieri seized control of the state, as one condottiero, Francesco Sforza, made himself the Duke of Milan in 1450. During the ages of the Taifa kingdoms of the Iberian peninsula, Christian knights like El Cid could fight for a Muslim ruler against his Christian or Muslim enemies. The Almogavars originally fought for the counts of Barcelona and kings of Aragon, but as the Catalan Company, they followed Roger de Flor in the service of the Byzantine Empire. In 1311, the Catalan Great Company defeated at the Battle of Halmyros their former employer, Walter V, Count of Brienne, after he refused to pay them, and took over the Duchy of Athens. The Great Company ruled much of central and southern Greece until 1388–1390 when a rival mercenary company, the Navarrese Company were hired to oust them. Catalan and German mercenaries also had prominent role in the Serbian victory over Bulgarians in the Battle of Velbuzd in 1330.

During the later Middle Ages, Free Companies (or Free Lances) were formed, consisting of companies of mercenary troops. Nation-states lacked the funds needed to maintain standing forces, so they tended to hire free companies to serve in their armies during wartime. Such companies typically formed at the ends of periods of conflict, when men-at-arms were no longer needed by their respective governments. The veteran soldiers thus looked for other forms of employment, often becoming mercenaries. Free Companies would often specialize in forms of combat that required longer periods of training that was not available in the form of a mobilized militia.
The Routiers formed a distinctive subculture in medieval France who alternated between serving as mercenaries in wartime and bandits in peacetime. The routiers were very destructive and became a significant social problem. After the Treaty of Brétigny ended the war between England and France in 1360, the French countryside was overrun by Free Companies of routiers while the French Crown lacked the necessary military and economic strength to put an end to their activities. To rid France of the rampaging mercenaries and to overthrow the pro-English King Pedro the Cruel of Castile, Marshal Bertrand du Guesclin was directed by King Charles V of France to take the Free Companies into Castile with the orders to put the pro-French Enrique de Trastámara on the Castilian throne. Guesclin's mercanaries were organized into the Big Companies and French Companies and placed a decisive role in putting Enrique on the Castilian throne in 1369, who styled himself King Enrique II, the first Castilian monarch of the House of Trastámara.
The White Company commanded by Sir John Hawkwood is the best known English Free Company of the 14th century. Between the 13th and 17th centuries the Gallowglass fought within the Islands of Britain and also mainland Europe. A Welshman Owain Lawgoch (Owain of the Red Hand) formed a free company and fought for the French against the English during the Hundred Years' War, before being assassinated by a Scot named Jon Lamb, under the orders of the English Crown, during the siege of Mortagne in 1378.

15th and 16th centuries: Swiss mercenaries were sought during the late 15th and early 16th centuries as being an effective fighting force, until their somewhat rigid battle formations became vulnerable to arquebuses and artillery being developed at the same time. The Swiss Guard in particular were employed by the Papal States from 1506 (continuing to serve today as the military of Vatican City).
It was then that the German landsknechts, colourful mercenaries with a redoubtable reputation, took over the Swiss forces' legacy and became the most formidable force of the late 15th and throughout the 16th century, being hired by all the powers in Europe and often fighting at opposite sides. Sir Thomas More in his Utopia advocated the use of mercenaries in preference to citizens. The barbarian mercenaries employed by the Utopians are thought to be inspired by the Swiss mercenaries.
A class of mercenaries known as the Gallowglass dominated warfare in Ireland and Scotland between the 13th and 16th centuries. They were a heavily armed and armored elite force that often doubled as a chieftain's bodyguard.
At approximately the same period, Niccolò Machiavelli argued against the use of mercenary armies in his book of political advice The Prince. His rationale was that since the sole motivation of mercenaries is their pay, they will not be inclined to take the kind of risks that can turn the tide of a battle, but may cost them their lives. He also noted that a mercenary who failed was obviously no good, but one who succeeded may be even more dangerous. He astutely pointed out that a successful mercenary army no longer needs its employer if it is more militarily powerful than its supposed superior. This explained the frequent, violent betrayals that characterized mercenary/client relations in Italy, because neither side trusted the other. He believed that citizens with a real attachment to their home country will be more motivated to defend it and thus make much better soldiers.
The Stratioti or Stradioti (Italian: Stradioti or Stradiotti; Greek: Στρατιώτες, Stratiotes) were mercenary units from the Balkans recruited mainly by states of southern and central Europe from the 15th until the middle of the 18th century. The stradioti were recruited in Albania, Greece, Dalmatia, Serbia and later Cyprus. Most modern historians have indicated that the Stratioti were mostly Albanians. According to a study by a Greek author, around 80% of the listed names attributed to the stradioti were of Albanian origin while most of the remaining ones, especially those of officers, were of Greek origin; a small minority were of South Slavic origin. Among their leaders there were also members of some old Byzantine Greek noble families such as the Palaiologoi and Comneni. The stratioti were pioneers of light cavalry tactics during this era. In the early 16th century heavy cavalry in the European armies was principally remodeled after Albanian stradioti of the Venetian army, Hungarian hussars and German mercenary cavalry units (Schwarzreitern). They employed hit-and-run tactics, ambushes, feigned retreats and other complex maneuvers. In some ways, these tactics echoed those of the Ottoman sipahis and akinci. They had some notable successes also against French heavy cavalry during the Italian Wars.