The Pokédex keeps track of the Pokémon you’ve seen and caught. Use the Pokédex to learn more about the different species of Pokémon and their Evolutions. Pokémon you’ve caught appear in color. Pokémon you’ve seen but haven’t caught appear as silhouettes; this includes Pokémon seen during encounters as well as at Gyms. Pokémon you’ve neither seen nor caught appear as their Pokédex number. 
To access the Pokédex:
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Pokémon GO Game GUIDE