Experience points are required for a character to Level Up. When a character participates in battle, they gain experience points. The amount of experience points gained depends mainly on the class; lower classes such as Trainees will gain the most while higher classes like Paladins will gain the least amount. The amount of experience gained also depends on how the battle went; no damage infliction results in very little, dealing damage will get you more, and killing the enemy will yield the most experience. Relative Class Power is also an important factor in some games. Also, when fighting a boss or a thief, you'll gain an increased amount of experience that does not adhere to some of the aforementioned conditions; Ex: If Marcus killed a boss early-game, he would obtain the same amount of experience the lower-tiered units would had they done the same. After 100 points are accumulated, the unit levels up to the next level. In the Tellius Series, you can also distribute Bonus Experience points as you see fit on the Base screen. The amount of bonus experience points you earn depends on certain circumstances from the previous chapter.
In earlier Fire Emblems (starting from Fire Emblem: Mystery of the Emblem, as the first two Fire Emblems do not give EXP when a staff or white magic is used), staff users gained a fixed amount of EXP based on what staff they used, and it was halved on promotion. However, in Fire Emblem: Radiant Dawn, the amount of experience gained is not reduced by promotion. In Fire Emblem: Shadow Dragon, Fire Emblem: Shin Monshō no Nazo ~Hikari to Kage no Eiyū~, and Fire Emblem Awakening, the amount of experience gained is reduced by the level of the healer.