A Portal Key 
is a token that corresponds to the Portal from which it was initially Hacked.
Portal Keys have a variety of uses, which include:
- Remotely checking the Portal's Resonator status, Mod status, and surroundings
- Remotely recharging the corresponding Portal's Resonators
- Using the corresponding Portal as the destination of a Link
- Acting as a keepsake, collectible, or trophy
While a Key is marked with a level, this number is the Portal's level and not the Key's level. Agents of any Access Level can use any Portal Key to its full extent.
Unlike other items, Portal Keys are not dropped from item rolls. Instead, community studies suggest that about 75% of standard Hacks, and about 50% of Glyph Hacks, produce a Key after other item drops are calculated. Hacking an unclaimed Portal increases the odds by about 10%.
If an agent Hacks a Portal and its Key is already in his or her Inventory, he or she will not receive another copy of that Key. Learning to circumvent this restriction is an important part of a new agent's training.
A Portal's Info Card within the Scanner, as well as the Android Wear Ingress companion, show the number of the Portal's Keys the agent's Inventory contains.
Remote Reference
Possessing a Portal Key enables an agent to remotely observe the Portal and its surroundings. From the Inventory, the agent can see the Portal's alignment, level, and Resonator health.
Tapping the Key's image moves the Scanner's focus to the corresponding Portal. The agent can then observe the surroundings, including nearby Portals, loose XM, and dropped items. By tapping any Portal in range, the agent can check its deployment and mod status. These functions can be useful for reconnaissance and surveillance purposes without leaving the Scanner.
Remote Recharging
Agents can recharge Portals remotely using the corresponding Portal Key. Tapping the Key from within the Inventory screen reveals a Recharge button, which operates similarly to Recharging a nearby Portal.
The range at which an agent can remotely recharge a Portal, and the XM penalty for Recharging remotely, is determined by his or her Access Level.
Remotely Recharging a Portal is the only game-affecting action that an agent can perform at a distance. By strategically collecting and checking Keys, an agent can aid his or her team over a longer distance than he or she could reasonably travel.
While possessing a Portal's Key enables an agent to Recharge it, the agent will only receive notifications if he or she owns the Portal or one of its Resonators. Agents who wish to remotely defend another agent's Portal must ensure that they maintain at least one Resonator on it, or they won't even know that the Portal has been attacked.
Throwing Links
Linking into a Portal requires its Key. While outbound Links can be thrown from a Portal without requiring its Key, the item is required for each destination Portal.
When a Link is successfully made, the destination's Portal Key is consumed and removed from the agent's Inventory. An agent who wishes to make and maintain a Field must therefore collect at least six total Keys - one to throw each of the three Links, and one to Recharge each of the three Portals.
Dedicated teams of agents often amass and distribute Portal Keys for distant or easily-defended Portals. These are used as anchors in long-distance Megafield projects, or to throw record-breaking Links. For example, Keys to the virtually inaccessible Guantanamo Bay Portal are often used for Megafields over America's East Coast.
Collector's Items
Portal Keys are often collected as curiosities, souvenirs, or memorabilia. Agents consider these Keys to be collector's items, not to be consumed for Links nor regularly Recharged.
Popular souvenir Keys include landmarks like the House of Parliament, historic sites like Normandy Beach, and significant movie locations like the Quality Cafe in Los Angeles. The Key proves that the agent visited the location, and acts as a virtual travelogue.
Some Keys have unique properties. The Keys of deleted Portals remain in agent's Inventories, acting as exclusive trophies. The NL-1331 vehicle has its own Key, which agents can use to track the van as it drives across America. The landmarks that define some Portals no longer exist, making their Keys a glimpse into the past.
Other Keys' significance comes from their Portals' relevance to in-game events. For example, the Andes Road Portal on Governors Island, New York, which held a Lightman Shard for nearly 12 months, is a memento of the early event.
Hacking Multiple Keys
If an agent has a given Portal Key in his or her Inventory and Hacks the corresponding Portal, the Hack will never produce a second copy of the Key. Removing the Key from the Inventory in any way will permit the agent to Hack another copy.
The following mechanisms will remove the Key from the Inventory and enable redundant Key Hacks:
- Dropping the Key. The pile of Keys can be picked back up after a sufficient number have been Hacked.
- Loading the Key into a Capsule. The Keys can be unloaded after the desired number has been acquired.
- Using the 'More' glyph while glyph hacking during the command phase, before the hacking glyphs are displayed.
Dropping the Key is the more common method, as the Portal's Info Card provides a Key button with a Drop shortcut. Loading the Key into a Capsule requires navigating through the Inventory screen multiple times.