Swiss mercenaries (Reisläufer) were notable for their service in foreign armies, especially the armies of the Kings of France, throughout the Early Modern period of European history, from the Later Middle Ages into the Age of the European Enlightenment. Their service as mercenaries was at its peak during the Renaissance, when their proven battlefield capabilities made them sought-after mercenary troops. There followed a period of decline, as technological and organizational advances counteracted the Swiss' advantages. Switzerland's military isolationism largely put an end to organized mercenary activity; the principal remnant of the practice is the Swiss Guard at the Vatican.
In William Shakespeare's Hamlet, Act IV, Scene 5, Swiss mercenaries are called "Switzers" (Switzer is actually what the Swiss were called in English until the 19th century, hence Switzerland).
During the Late Middle Ages, mercenary forces grew in importance in Europe, as veterans from the Hundred Years War and other conflicts came to see soldiering as a profession rather than a temporary activity, and commanders sought long-term professionals rather than temporary feudal levies to fight their wars. Swiss mercenaries (Reisläufer) were valued throughout Late Medieval Europe for the power of their determined mass attack in deep columns with the pike and halberd. Hiring them was made even more attractive because entire ready-made Swiss mercenary contingents could be obtained by simply contracting with their local governments, the various Swiss cantons—the cantons had a form of militia system in which the soldiers were bound to serve and were trained and equipped to do so. Some Swiss also hired themselves out individually or in small bands.