The second generation introduced 100 new Pokémon.
These Pokémon debuted in Pokémon Gold and Silver, which were released in Japan in 1999 for the Game Boy Color. The new species were numbered from Chikorita at National Pokédex position 152 through Celebi at position 251.
Generation II expanded the series with the Johto region, a day-and-night system, Pokémon breeding, held items, and the Dark and Steel types. After completing the Johto adventure, players could also revisit the Kanto region from the first generation.
The total can be confused with the number of Pokémon in the original games. Generation I introduced 151 species; adding 100 from Generation II produced a collection of 251. Pokémon Crystal later served as the enhanced version of the second-generation pair.