Mercenary
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. They were known for cutting off the heads of dead or captured enemies, and according to Commines they were paid by their leaders one ducat per head.
In Italy, during inter-family conflicts such as the Wars of Castro, mercenaries were widely used to supplement the much smaller forces loyal to particular families. Often these were further supplemented by troops loyal to particular duchies which had sided with one or more of the belligerents.

17th and 18th centuries: During the 17th and 18th centuries, extensive use was made of foreign recruits in the now regimented and highly drilled armies of Europe, beginning in a systematized way with the Thirty Years' War. Historian Geoffrey Parker notes that 40,000 Scotsmen (about fifteen percent of the adult male population) served as soldiers in Continental Europe from 1618 to 1640.
After the signing of the Treaty of Limerick (1691) the soldiers of the Irish Army who left Ireland for France took part in what is known as the Flight of the Wild Geese. Subsequently, many made a living from fighting in continental armies, the most famous of whom was Patrick Sarsfield, who, having fallen mortally wounded at the Battle of Landen fighting for the French, said "If this was only for Ireland".
The brutality of the Thirty Years' War, in which several parts of Germany were ransacked by the mercenary troops, and left almost unpopulated, led to the formation of standing armies of professional soldiers, recruited locally or abroad. These armies were active also in peacetime. The formation of these armies in the late 18th century led to professionalization and standardization of clothing (uniforms), equipment, drill, weapons, etc. Since smaller states like the Dutch Republic could afford a large standing army, but could not find enough recruits among its own citizens, recruiting foreigners was common. Prussia had developed a form of conscription, but relied in wartime also on foreign recruits, although the regulations stated that no more than one third of the recruits were to be foreign. Prussian recruiting methods were often aggressive, and resulted more than once in conflicts with neighbouring states. The term mercenary gained its notoriety during this development, since mercenaries were—and now are—often seen as soldiers who fight for no noble cause, but only for money, and who have no loyalty than to the highest bidder, as opposed to the professional soldiers who takes an oath of loyalty and who is seen as the defender of the nation.
The mercenary soldiers thus fell out of favour and was replaced by the professional soldier. To augment the army, major European powers like France, Britain, the Dutch Republic and Spain contracted regiments from Switzerland, the Southern Netherlands (modern day Belgium), and several smaller German states. About a third of the infantry regiments of the French Royal Army prior to the French Revolution were recruited from outside France. The largest single group were the twelve Swiss regiments (including the Swiss Guard). Other units were German and one Irish Brigade (the "Wild Geese") had originally been made up of Irish volunteers. By 1789 difficulties in obtaining genuinely Irish recruits had led to German and other foreigners making up the bulk of the rank and file. The officers however continued to be drawn from long established Franco-Irish families. During the reign of Louis XV there was also a Scottish (Garde Écossaise), a Swedish (Royal-Suédois), an Italian (Royal-Italien) and a Walloon (Horion-Liegeois) regiment recruited outside the borders of France. The foreign infantry regiments comprised about 20,000 men in 1733, rising to 48,000 at the time of the Seven Years' War and being reduced in numbers thereafter.

The Dutch Republic had contracted several Scots, Swiss and German regiments in the early 18th century, and kept three Scots, one Walloon, and six Swiss regiments (including a Guard regiment raised in 1749) throughout the 18th century. The Scots regiments were contracted from Great Britain, but as relations between Britain and the Republic deteriorated, the regiments could no longer recruit in Scotland, leading to the regiments being Scots in name only until they were nationalized in 1784. Patrick Gordon, a Scottish mercenary fought at various times for Poland and Sweden, constantly changing his loyalty based on who could pay him the best, until he took up Russian service in 1661. In August 1689, during a coup d'état attempt in Moscow against co-tsar Peter the Great led by the Sophia Alekseyevna in the name of the other co-tsar, the intellectually disabled Ivan V, Gordon played the decisive role in defeating the coup and ensuring Peter's triumph. Gordon remained one of Peter's favorite advisers until his death.
The Spanish Army also made use of permanently established foreign regiments. These were three Irish regiments (Irlanda, Hiberni and Ultonia); one Italian (Naples) and five Swiss (Wimpssen, Reding, Betschart, Traxer and Preux). In addition one regiment of the Royal Guard including Irishmen as Patten, McDonnell and Neiven, was recruited from Walloons. The last of these foreign regiments was disbanded in 1815, following recruiting difficulties during the Napoleonic Wars. One complication arising from the use of non-national troops occurred at the Battle of Bailén in 1808 when the "red Swiss" (so-called from their uniforms) of the invading French Army clashed bloodily with "blue Swiss" in the Spanish service.
During the American Revolutionary War, the British government hired several regiments from German principalities to supplement the Army. They became known to revolutionaries as Hessians and were portrayed by propagandists as mercenaries. However, they were auxiliaries and do not meet the definition of mercenary.

19th–21st centuries: During the South American wars of independence from Spain, the British Legions from 1817 onward fought for General Simón Bolívar. Some of the British Legionaries were liberal idealists who went to South America to fight in a war for freedom, but others were the more classic mercenaries, mostly unemployed veterans of the Napoleonic wars, who fought for money. In South America, especially in Colombia, the men of the British Legions are remembered as heroes for their crucial role in helping end Spanish rule. During the First Carlist War, the British government suspended the Foreign Enlistment Act to allow the recruitment of a quasi-official British Auxiliary Legion under George de Lacy Evans, which went to Spain to fight for Queen Isabel II against the followers of Don Carlos, the pretender to the Spanish throne.
The Atholl Highlanders, a private Scottish infantry regiment of the Duke of Atholl, was formed in 1839 purely for ceremonial purposes. It was granted official regimental status by Queen Victoria in 1845 and is the only remaining legal private army in Europe.
Turkey and Azerbaijan have deployed Syrian mercenaries during the 2020 Nagorno-Karabakh war.
Syrian mercenaries are being deployed by Russia, with expected numbers ranging from hundreds to up to 40,000 fighters ultimately expected to take part. Wagner mercenaries are active in the Syrian civil war and sledgehammered a Syrian man to death. Mozart Group is also active in Ukraine.

East Asia: 

Warring States: Mercenaries were regularly used by the kingdoms of the Warring States period of China. Military advisers and generals trained through the works of Mozi and Sun Tzu would regularly offer their services to kings and dukes.
After the Qin conquest of the Warring States, the Qin and later Han Empires would also employ mercenaries – ranging from nomadic horse archers in the Northern steppes or soldiers from the Yue kingdoms of the South. The 7th-century Tang dynasty was also prominent for its use of mercenaries, when they hired Tibetan and Uyghur soldiers against invasion from the Göktürks and other steppe civilizations.

15th to 18th centuries: The Saika mercenary group of the Kii Province, Japan, played a significant role during the Siege of Ishiyama Hongan-ji that took place between August 1570 to August 1580. The Saikashuu were famed for the support of Ikkō Buddhist sect movements and greatly impeded the advance of Oda Nobunaga's forces.
Ninja were peasant farmers who learned the art of war to combat the daimyō's samurai. They were hired out by many as mercenaries to perform capture, infiltration and retrieval, and, most famously, assassinations. Ninja possibly originated around the 14th century, but were not widely known or used till the 15th century and carried on being hired till the mid 18th century.