The first knights were not westerners but rather from Sassanid Persia. Ammianus Marcellinus, a Roman general and historian, who served in the army of Constantius II in Gaul and Persia, fought against the Persians under Julian the Apostate and took part in the retreat of his successor, Jovian. He describes the Persian knight as:

"All their companies clad in iron, and all parts of their bodies were covered with thick plates, so fitted that the stiff joints conformed with those of their limbs; and forms of the human faces were so skillfully fitted to their heads, that since their entire bodies were covered with metal, arrows that fell upon them could lodge only where they could see a little through tiny openings opposite the pupil of the eye, or where through the tips of their noses they were able to get a little breath."

"The Persians opposed us serried bands of mail-clad horsemen in such close order that the gleam of moving bodies covered with closely fitting plates of iron dazzled the eyes of those who looked upon them, while the whole throng of horses was protected by coverings of leather."

The knight depicted in the Persian relief here is also thought to be of royal nobility as well.

In the chaos and danger of post-Roman Western Europe, the population had very little organized governmental protection from brigands and conquering warbands. Knowing there was safety in numbers, local lords (who could afford it) gathered around them young, fighting-age men to fend off rebellious vassals or conquering neighbors. These men, in turn, were rewarded with war booty for their service and loyalty. Soon, grants of land were made so the young soldiers could receive an income from those lands and afford the high cost of outfitting themselves with the accoutrements of war, such as horses, armor, and weapons. The era of the medieval knight had begun.

During the middle ages, the term knight referred to a mounted and armoured soldier. Originally, knights were warriors on horse-back, but the title became increasingly connected to nobility and social status, most likely because of the cost of equipping oneself in the cavalry. Knighthood eventually became a formal title bestowed on those noblemen trained for active war duty. At the beginning of the knight’s history, horsed soldiers were not so important in war; instead, foot soldiers were the main fighting force in all North European armies. Later, as invaders kept attacking the lands that the Roman Empire had, until recently, occupied, the knight became more and more important in obtaining victory. One of the reasons being that the invading armies had cavalry (mounted troops) and the northern European foot soldiers found it increasingly difficult to maneuver against their superior foes; as a result, knights were greatly needed in battle, and the knights in shining armor that we keep in mind today were born.

Knighthood was closely connected with the feudal system. Originating largely in what later became known as France, this was a social organisation in which warfare and the protection of the common people became the specialised skill of a select group. Instead of having them paid in cash — of which everyone, even the monarch, was short — they were paid in land. These rather extensive pieces of land were the fiefs. Though a fief did not have to be land — it could be any payment — it is generally thought of as the land that the knights were given as payment for service to the king. The knights were economically supported by peasants who worked to produce food and ideologically supported by the contemporary church.

Sometimes these knights were the noble themselves and sometimes men they hired, because noblemen were disinclined or unable to fight. In times of war or national disorder the monarch would typically call all the knights together to do their annual service of fighting. This could be against internal threats to the nation or in defensive and offensive wars against other nations.

As time went by, monarchs began to prefer standing (permanent) armies because they could be used for longer periods of time, were more professional and were generally more loyal; partly because those noblemen who were themselves knights, or who sent knights to fight, were prone to use the monarch's dependency on their resources to manipulate him. This move from knights to standing armies had two important outcomes: the regular payment of "scutage" to monarchs by noblemen (a money payment instead of actually going to fight as a knight) which would strengthen the concept and practice of taxation, and a general decrease in military discipline in knights, who became more interested in their country estates and chivalric pursuits, including art and sport.

Originally, knighthood could be bestowed on a man by any knight, but it was generally considered prestigious to be dubbed knight by the hand of a monarch or royalty; the monarch eventually acquired the exclusive right to confer knighthoods. By about the late 13th century, partly in conjunction with the focus on courtly behavior, a code of conduct and uniformity of dress for knights began to evolve. Knights were eligible to wear a white belt and golden spurs as signs of their status. Moreover, knights were also required to swear allegiance — either to a liege lord or to a military order.