The Overshield technology was developed by the Covenant to enhance their Elite warriors' shields. In Halo: Combat Evolved and Halo 2, overshields boost a player's shields to 300%. In Halo 3, however, these overshields only boost a player's shields to 200% (although MLG's playlists have a Custom Powerup Overshield that is of 300% strength). Overshields can be used by any Spartan (in Mark V or VI MJOLNIR armor) or Elite who might come upon it.
It takes approximately three seconds to charge once picked up. During this time, the player is invincible. The player's normal shield does not take damage until the overshield is completely depleted (although any damage overlapping from something destroying the overshield, i.e. a grenade's explosion, will also do damage to the player's normal shield). If the overshield is picked up when the player's shield is below 100%, the player receives a full charge and the effects of the overshield stay.
In the single player Campaign, the overshield is reduced only when the player takes damage. In Multiplayer, it gradually depletes over time. The overshield can only be found in Campaign in some levels of Halo: Combat Evolved. Halo 3 includes a piece of equipment in Campaign, invincibility, which has a shield-strengthening effect, though it only lasts 10 seconds. In Halo 3: ODST, all hostiles next to a Covenant Engineer are given an overshield; in the case of Brutes, this does not enhance their armor's shields, it stacks on top of it. Towards the end of the campaign, the player (as the Rookie) accompanies a friendly Engineer named Vergil, and staying near to him will grant overshield abilities as well, though it isn't quite clear how it effects the player, as the HUD for Orbital Drop Shock Troopers does not include a shield meter, as their armor lacks shields.
In Halo: Combat Evolved, the overshield is a glowing red orb of energy encased in a clear glass cube. In Halo 2, both the orb and cube are gold in color. In Halo 3, the orb, again glowing red, appears to be held together by several small magnets. After walking over an overshield in Halo: Combat Evolved or Halo 2, the person's HUD shield display changes color from blue to red then green. In Halo 3, the HUD shield display color changes from blue to green. Each color indicates one layer of shields.
While wearing an overshield in Halo 2 and Halo 3, the MJOLNIR armor is surrounded by a glowing force field which is easily seen even when wearing the Active Camouflage. This does not happen in Halo: Combat Evolved.
On the HUD, the shield monitor shows a light green for a fully charged overshield, red for the second level, and the usual blue/purple (depending on player model) for the normal shields. The Halo: Combat Evolved overshield shows yellow/green at the highest level, rather than green. In Halo 3, the overshield only raises your shield level by one level, showing green as fully charged.
Overshields effectively triple the strength of the MJOLNIR and Elite armor's shields in Halo 2 and double in Halo 3. The shields can now protect the person from heavy weapons such as Rocket Launchers, Fuel Rod Cannons, Sniper Rifles, Fragmentation Grenades, and even Plasma Grenades. Also, while the shields are charging, you are immune to conventional damage, but will still be vulnerable to assassination, Guardians, and falling to your death.
Due to the bright glow of the overshields, the wearer is made more visible to other players, often drawing unwanted attention. In Multiplayer, the overshield slowly loses its strength over prolonged periods of time, unless the player is a Juggernaut, VIP, or Grifball carrier. An overshield can be fully depleted by an overcharged Plasma Pistol shot. Also, many players place traps near/on the Overshield, such as throwing Fragmentation Grenades or placing Trip Mines near it.
Note: The glow does not appear in Halo: Combat Evolved, and therefore has no disadvantages. The glow was probably added in the later games to balance the powerup.