Generation I of the Pokémon video game series introduced 151 Pokémon species.
These species first appeared in Pocket Monsters Red and Green, released in Japan for the Game Boy in 1996. The National Pokédex numbers run from Bulbasaur at No. 001 through Mew at No. 151.
The original games presented the first complete collection of creatures to catch, train, battle, and trade. Players could not normally obtain every species in one cartridge, because Red and Green contained version-exclusive Pokémon that encouraged trading through the Game Link Cable.
A common mix-up is treating Mew as a regular story encounter. Mew is No. 151 and was hidden in the original games, while the games’ main in-game Pokédex sequence ends with Mewtwo at No. 150. Later generations expanded the Pokédex with many more species.