In Generation III and onwards each Pokémon has Abilities inherent to their species. Abilities have a variety of effects; some examples include helping the Pokémon with the Ability, lowering the opponent's stats, preventing status changes, changing the weather, restoring HP under certain conditions, transforming the Pokémon, and hindering the Pokémon with the Ability.
A Pokémon species with only one Ability will always have that Ability, unless it is temporarily changed during a battle by a means such as Skill Swap. A Pokémon species with two Abilities has a 50% chance of having either, depending on its personality value. A dual-Ability species will, due to the retention of the personality value as the Pokémon evolves, keep the first Ability or second Ability of its species on evolution, even if the Abilities are different for different stages. For example, a Horsea with Swift Swim will always have Poison Point if evolved into Seadra, as Swift Swim is Horsea's first Ability, and Poison Point is Seadra's. If evolved further into Kingdra, the Seadra's Ability will return to Swift Swim, as this is Kingdra's first Ability. A Horsea with Sniper, however, will retain this Ability throughout its evolutionary line, as this is the second Ability of Horsea, Seadra, and Kingdra.
Some Abilities introduced in Generation IV were assigned to Pokémon introduced in previous generations, changing them from being single-Ability Pokémon into dual-Ability Pokémon. When transferred through Pal Park, a Pokémon will keep its Generation III Ability, however, upon evolution, the Ability may change into the other Ability for the species, depending on the personality value of the Pokémon. In this way, a Horsea with Swift Swim caught in Hoenn can become a Seadra with Sniper, but only if Horsea's personality value dictates it.
Most Pokémon were given Hidden Abilities in Generation V. In this way, some Pokémon now have the ability to have one of three Abilities—two dependent on their personality value if caught normally, and one for Pokémon obtained in certain ways, such as from the Pokémon Global Link.
The Ability Capsule, introduced in Generation VI, allows all Pokémon (except for Greninja and Zygarde) to switch between their first and second Abilities. If a Pokémon has only one Ability, or currently has its Hidden Ability, they are unaffected by the Ability Capsule. The Ability Capsule will never switch a Pokémon's Ability to its Hidden Ability.