Dark and Steel arrive, and Pokémon stops being broken.
Gold, Silver, Crystal (and the HeartGold/SoulSilver remakes) · Released 1999
Johto is the region of Pokémon Gold & Silver — the sequel that introduced breeding, shiny Pokémon, held items, day/night cycles, and the two most important type additions in franchise history: Dark and Steel. These were deliberately added to counter the dominant Psychic-type of Gen 1, and their introduction rebalanced the entire type chart. Johto also continued Kanto's map, letting players revisit all 8 Gen 1 gyms after beating the Elite Four.
Johto is based on Japan's Kansai region, specifically Kyoto and Nara. The Ruins of Alph, Bell Tower and Burned Tower reference real Japanese temple architecture and shrine ruins. Lugia and Ho-Oh were the first pair of 'legendary birds in flagship boxart' roles — a formula Game Freak has used in every generation since.
Gold & Silver's Johto map + revisitable Kanto map made Gen 2 the largest Pokémon game ever shipped to this day — the developers had to use a custom data compression routine to fit both regions on a single Game Boy cartridge.
Gold and Silver made more consequential changes to the rules of Pokémon than any generation since. Splitting the Special stat into Special Attack and Special Defence ended Psychic's dominance overnight. Adding the Dark and Steel types gave the chart answers to Psychic and Dragon respectively.
It also introduced held items, breeding, genders, shiny Pokémon and a real-time clock driving day and night cycles. Almost every system a modern player takes for granted, other than abilities and natures, arrived here.
The one thing Johto is fairly criticised for is its level curve and distribution: many Johto Pokémon appear late or rarely, so a first playthrough tends to lean heavily on Kanto species despite 100 new ones being available.
Johto's 100 additions are unusually top-heavy. Blissey remains one of the best special walls ever printed, Tyranitar's Sand Stream defined weather-based team building for years, and Scizor and Steelix rescued two mediocre Gen 1 Pokémon outright.
The Johto starters have fared less well. Typhlosion and Feraligatr are serviceable, and Meganium is widely regarded as one of the weakest starters in the series — a rare case where the community consensus is close to unanimous.
The Dark and Steel types, the split of Special into Special Attack and Special Defence, held items, breeding, genders, shiny Pokémon, and a real-time day and night cycle. It is the most mechanically significant generation in the series' history.
100, numbered #152 Chikorita through #251 Celebi — the smallest new-Pokémon count of any generation, though Gold and Silver compensated by making the existing 151 far more interesting through new evolutions and typings.
Primarily to rebalance the type chart. Psychic had almost no counterplay in Generation 1, and Dark was created to answer it. Steel resists ten types and gave the game a defensive answer to the Dragon and Fairy-less top tier of the era.