Cuirassiers were mounted soldiers with firearms originating in 16th-century Europe. In hindsight, they constituted a transitional form of cavalry unit between medieval armored knights and latter-day cavalry. The name comes from cuirass, the breastplate armor they used.

The first cuirassiers did not appear very different from the medieval knights; they wore full-body armor and the only things that separated them from the knight were riding boots and the use of wheel-lock pistols, in addition to lances and swords.

Cuirassiers wore armor long after it had become superfluous in the face of ever-increasing firearm use. However, the size of the armor gradually decreased so that, by the end of the 17th century, it comprised only a breastplate (the cuirass or Plastron), and the backplate, or Carapace and the helmet.

The first recorded cuirassiers formed as 100-man strong regiments of Austrian kyrissers in 1484 to serve the future Holy Roman Emperor Maximilian. Oliver Cromwell formed groups of cuiraissiers in the English Civil War. The French military introduced their own cuirassiers in 1666. By 1705, the Holy Roman Emperor's personal forces in Austria included twenty cuirassier regiments. Imperial Russia formed its own cuirassier regiments in 1732, including a Leib Guards regiment. The Russian cuirassier forces took part in the war against Turkey in 1736.

Cuirassiers played a prominent role in the armies of Frederick the Great of Prussia and of Napoleon of France. The latter increased the amount of cuirassier regiments to 14 by the end of his reign.

Eventually most cuirassier regiments evolved into other forms of cavalry such as lancers, hussars, or particularly dragoons (essentially mounted modern infantry). Some cavalry regiments continue to use cuirasses as part of their parade paraphernalia and in other formal functions. The term cuirassiers has become mainly an honorific term, retained from the regiment's past if retained at all.