Swiss mercenaries (Reisläufer) are 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 is at its peak during the Renaissance, when their proven battlefield capabilities make them sought-after mercenary troops.

There follows a period of decline, as technological and organizational advances counteract the Swiss' advantages.

Switzerland's military isolationism largely puts 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 nineteenthth century).