Once the player obtains a mod, this mod is unlocked in their collection, and can be slotted into any appropriate piece of gear. The player does not spend any mods when they are installed, but slotting a mod or swapping it with another one costs Glimmer. Multiple mods can be installed into a piece of armor, a single mod can be installed into a weapon, but some items do not support mods at all.
Year 2 introduced a change to Weapon Mods. Previously, those were used to determine the type of damage done by the weapon: Kinetic, Arc, Solar or Void. Now, these mods slightly alter the weapon's performance, like Backup Mag, which increases a gun's magazine, or Icarus Grip, which improves accuracy while airborne. Year 4 brought Adept mods, which can be used only on Adept weapons obtained by going Flawless in the Trials of Osiris or by completing Grandmaster Nightfall: The Ordeal.
Year 3 introduced massive changes to armor, mods included. Each armor piece has an elemental type that determines which armor mods can be installed into it. Armor mods are divided into 7 types: General, Head, Arms, Chest, Legs, Class Item and Combat Style. General and Combat Style Armor Mods can be slotted into a designated slot on any piece of armor, other types can be inserted only in designated slots on the specified armor piece. Raid armor has an additional socket for mods from that raid only. Year 3 armor's Combat Style socket is replaced with Legacy, which accepts the same mods, as well as Nightmare Hunt, Last Wish and Garden of Salvation mods.
Armor pieces also have a certain energy charge value, which can be increased by Masterworking them. Each armor mod has an energy cost, which displays how much of an armor's charge this mod will take. For example, Ashes to Assets is a Solar Helmet Armor Mod with an energy cost of 3. It can be installed only into a designated socket in a Solar type Helmet which has at least 3 free charges.
Starting from Year 4 Ghost Shells lost their perks in exchange for mod sockets. Like armor, Ghost Shells can be upgraded, which increases their energy capacity and allows to socket mods, but without elemental affinity. There are four mod sockets on each Ghost Shell: Experience, Tracking, Economic, and Activity. The last one is initially unavailable and unlocks only after Masterworking the Ghost Shell.
Season of the Undying brought Seasonal Artifacts into the game. By gaining experience, the player can unlock 12 out of 25 available seasonal mods in the Artifact. These mods are removed from the game at the start of a new Season, unless the next Season's Artifact contains the same mod, but the player will have to earn it again.
Previously, there was an exchange system, but as of now, the mods can be obtained from these sources:
- Rank-up packages from most of the vendors;
- Banshee-44 sells 2 mods, his stock changes daily;
- Some mods can be obtained by doing various activities, like Raids.
Due to the amount of mods in the game, the list of mods is separated by types:
- Weapon Mods
- General Armor Mods
- Helmet Armor Mods
- Arms Armor Mods
- Chest Armor Mods
- Leg Armor Mods
- Class Item Mods
- Seasonal Mods
Legacy and Raids
- Vault of Glass Armor Mods
- Deep Stone Crypt Raid Mods
- Garden of Salvation Raid Mods
- Last Wish Raid Mods
- Nightmare Mods
- Obsolete Mods (archive)