"Generation I" was the first Generation of the Pokémon series. Generation I introduced 151 Pokémon, labelled, as expected, 001 through 151 in the National Poké Dex.
Generation I consists of four games: Pokémon Red, Blue, Green and Yellow Versions. The first of these games released were Pokémon Red and Green Versions, which were released in Japan in 1996. Pokémon Blue Version was released in Japan in 1999, and acted as the definitive game of the first generation; fixing various glitches and bugs, while upgrading the graphics and sound quality. When the series was brought to international regions, the games were based heavily off of Pokémon Red and Green Versions, but used the scripting and engine of Pokémon Blue Version. These versions became known as Pokémon Red and Blue Versions. The final game released for Generation I was Pokémon Yellow Version, which drew heavy inspiration from the then-new Pokémon anime franchise. Unlike any other core game in the franchise, Yellow forced players to use a Starter Pokémon in Pikachu.
Generation I received a single side game in Pokémon Stadium, though there exist two versions of this game in Japanese and International versions. The Japanese version was incomplete at the time of release, and the international version served as a more balanced and complete version of the game.