Key military developments occurred in the Early Modern period, during which armies evolved from troops of medieval knights to Napoleon's mass levies. Firearms impelled change, necessitating new battlefield tactics and fundamentally altering siege and naval warfare. The size and cost of military forces expanded enormously, and new standing armies underpinned the growing absolutist power of princes. Academic experts from both sides of the Atlantic review these developments, discussing the medieval legacy, Spain, the Ottoman Turks, the Thirty Years War, Prussia, the ancien régime and the Napoleonic Wars, together with sea power, the American Revolution and warfare outside the West.