When gathering items at PokéStops, you may find a Pokémon Egg. Place the Egg in an incubator, and the Egg will hatch into a Pokémon as you walk. To hatch an Egg:


Each Egg displays the number of kilometers you must walk for the Egg to hatch. In general, the more kilometers required, the rarer the Pokémon that will hatch. A notification will appear when the Egg has hatched. An incubator can only hold one Egg at a time, and you’ll find one infinite-use incubator in your inventory. To incubate and hatch multiple Eggs at the same time, you can purchase additional incubators in the Shop. Tip: Turn on Adventure Sync to track your kilometers walked even while the Pokémon GO app is closed. If Adventure Sync is turned off, you’ll need to keep the Pokémon GO app open while you walk to have your distance walked count toward hatching your Eggs.
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Pokémon GO Game GUIDE