Experience is primarily accrued by defeating enemies and completing missions.
Experience Requirements Table
This table indicates the experience requirements for each level in both City of Heroes and City of Villains.
Experience Rewards Table
These are the base experience reward values for each rank and level of critter, as well as missions. Some critters have a modifier that increases or decreases the amount of XP rewarded depending upon the difficulty of the critter.
Experience Distribution and Team Bonuses
The experience that a character gets while on a team is affected by the team's size, by his level relative to the enemies, and by his level relative to his teammates. Larger teams get a higher XP multiplier, but the rewards also get split more ways. Lower-level characters get more XP from each critter (since the critters are comparably tougher), but the game also gives greater-than-average shares of XP to the higher-level characters in the team (since they are presumed to be doing most of the work).
Mission completion experience is affected only by the character's level relative to the mission's. The full amount is awarded to each team member individually, with no split or weighting.