Levels are featured predominantly in the Pokémon games. They are determined by how much experience that Pokémon has. A Pokémon's level will range from 1 to 100. When a Pokémon gains a level, its stats increase by a small amount. It may also learn a new move or evolve.
A Pokémon's level will also affect its happiness. Using Rare Candies will also raise happiness, though for a long time people thought it did not, due to a Pokémon's happiness also being raised by the large amount of walking that a player must do in long grass to encounter wild Pokémon or recharge such items as the Vs. Seeker.
In Generations III and IV, when a Pokémon has reached level 100, and has not gained maximum EV's, it cannot continue gaining Effort Points through battle. Vitamins can still be used to raise EV's. In Generations I and II, the player can use the box trick. In V stats are recalculated after every battles, so a level 100 Pokémon gains and applies EVs normally.
By exploiting the old man glitch in Generation I, a Pokémon can be acquired at a level higher than 100, and can continue to be leveled up with Rare Candies; however, whenever this Pokémon gains any amount of experience, its level will revert to 100, and if a Rare Candy is fed to a level 255 Pokémon, it will revert to level 0.
In Generations I and II, Pokémon assigned to the "Medium Slow" experience formula (1,059,860 Exp. at level 100) were unavailable at a level below 2 due to the "Medium Slow" formula generating a negative Exp. value at level 1; this resulted in such a level 1 Pokémon jumping straight to level 100. Pokémon eggs, therefore, hatched at a standard level of 5. The Generation III overhaul addressed this error; however, eggs still hatched at level 5, likely to maintain continuity. Generation IV finally set all hatched eggs to level 1.
When a Pokémon is obtained in a trade, it oftentimes will not obey the player's commands if it is at too high a level. The Pokémon will either ignore orders and use a different move, do nothing at all and loaf around, or go to sleep. This can be corrected if the player has the proper badges. Having less than two badges means that no traded Pokémon whose level is above 10 will obey the player, while having all eight makes all Pokémon obey the player. The specific badges that cause a rise in the level of obeying Pokémon can be found on their page, though they are typically the second, fourth, sixth, and eighth badges obtained.