Monitors are highly advanced artificial intelligence constructs, or ancillas, created by the Forerunners well over 125,000 years ago to occupy special positions in Forerunner society. Some monitors are tasked with servicing and maintaining the Halo installations and ensuring that the virulent Flood stay imprisoned. Other monitors oversee the maintenance and function of Line installations. Still others were used to provide security on the Forerunner capital and a limited number were used in combat by Warrior-Servants. Monitors are extremely intelligent, yet are completely devoted to their original function, and are zealous about containing Flood; monitors have been known to turn violently on their allies if they should attempt to violate their containment protocols. The Covenant refer to monitors as Oracles; they revere the "Oracles" for their wisdom and believe that they are key to the fulfillment of the Great Journey.
A monitor's body consists of a roughly spherical shape that is concave on three sides, with an illuminated photoreceptor located on the front of the orb. A monitor's silvery metal covering is reminiscent of other Forerunner constructs, though their spherical shapes stand in contrast to the polygonal, angular shapes commonly seen in other Forerunner creations.
Monitors' photoreceptors vary in color, including blue, red, orange and green. However, some, if not all monitors' photoreceptors can vary in color; for example, 343 Guilty Spark's photoreceptor was generally blue, though it turned red when he was in a combative state. 2401 Penitent Tangent's and 686 Ebullient Prism's photoreceptors were perpetually red, despite no evidence of combat-related behavior. The Warden's photoreceptor was dull green.
343 Guilty Spark and 2401 Penitent Tangent both possessed high-pitched, tinny voices. 049 Abject Testament's voice also had a tinny quality, but was notably deeper.
 Function and behavior
Each Halo installation has at least one monitor assigned to it, which oversees all operations on the ring. Line Installation 1-4 had only one known monitor, 686 Ebullient Prism. Numerous monitors were used to provide security for the Forerunners' Capital Court. Monitors were sometimes used in combat by Warrior-Servants. The Warden was a venerable monitor that was used to both detain accused criminals and to defend them in court.
Most known monitors are identified by a series of numbers followed by a name. The monitors of the Halo installations have identification numbers comprised of seven raised to the power of the monitor's installation's number minus one (7n-1). The names appear to consist of an adjective describing a negative emotion followed by a technological term. In Forerunner communication logs, Halo monitors are identified by their installation's number, a dash, and their own number; for example, 343 Guilty Spark, the monitor of Installation 04, is identified on such logs as "04-343."
Each Halo's monitor commands the Sentinels of the installation they oversee. The latter two are only activated if a catastrophic Flood outbreak occurs. The only known monitor of a Line installation, 686 Ebullient Prism, also commanded its installation's Gatherers.
Monitors are extremely intelligent, but are also devoted to their original functions and to protocol; this results in a zealous attitude toward containment of the Flood and, in the case of Halo installations' monitors, an overeagerness to activate the Halo Array. Monitors frequently cite protocols as explanations for their actions, no matter how impractical; this has been known to confuse or annoy human and Covenant forces who have interacted with them. It is possible that a Halo's monitor sees its protocols as the only options in a situation; during the Battle of Installation 04, 343 Guilty Spark could have taken numerous precautions to prevent the Flood from spreading across the installation, but the only action he ever considered was the activation of the Halo.[note 1] During the Battle of Line Installation 1-4, 686 Ebullient Prism frequently informed his human and Covenant prisoners that they would be killed and dissected in an effort to find immunity to the Flood, showing no concern for the subjects' well-being. The monitors' adherence to protocol is also shown in their seemingly standardized method of introducing themselves: "Greetings. I am [Number] [Name]. I am the monitor of Installation [Number]."
Similarly to a human "dumb" AI, monitors seem to have nearly limitless information of their own expertise, yet apparently lack information that does not pertain to their defined purpose. During the Battle of Installation 04, and later on Installation 04B, 343 Guilty Spark seemingly addressed SPARTAN John-117 as the IsoDidact. According to 343 Guilty Spark, a Monitor's knowledge is intentionally limited by design so that as little as possible would be divulged should it be captured by the Flood.
Monitors have a remarkable resistance to small arms fire, though they will eventually succumb after taking extensive damage. On Installation 04B, John-117 was able to destroy 343 Guilty Spark only with multiple discharges from a Spartan Laser, - a weapon that can destroy tanks with a single discharge - demonstrating that monitors can sustain damage from high-powered weaponry. During the Battle of Line Installation 1-4, Reff 'Talamee severely damaged 686 Ebullient Prism's photoreceptor and casing with an Aggressor's weapon; the monitor was eventually destroyed by its installation's main weapon. Should a monitor be damaged, it is capable of self-repair.
Though monitors are assisted by other constructs in the event of a Flood outbreak, the monitors also have their own offensive capabilities. Their primary armament is a red-colored directed energy beam that is capable of depleting the shields of a Mark VI MJOLNIR armor suit in a single blast, as well as mortally wounding a less well-armored human. This weapon can also kill a Flood combat form almost instantaneously.
Monitors also possess two types of utility beams. One of these beams allows a monitor to manipulate and levitate objects. The second is a blue beam that can unlock doors in Forerunner installations and can repair some technology.
Monitors have access to their assigned installation's translocation grid, allowing them to quickly move across the span of the installation. For long-distance travel outside their installations, monitors are equipped with impulse drives.
 Known monitors and their assignments
- 03-049 Abject Testament - Installation 03
- 04-343 Guilty Spark - Installation 04, later Installation 04B
- 05-2401 Penitent Tangent - Installation 05
- 686 Ebullient Prism - Line Installation 1-4
- The Warden - Detained and provided defense for Forerunners accused of crime before the Capital Court.
- In Halo: Combat Evolved and Halo 3, monitors' eyes are branded with the Marathon symbol. The symbol has been altered in Halo: Reach and Halo: Combat Evolved Anniversary for copyright reasons.
- During the cinematic commentary on the Halo 3 Essentials DVD, Martin O'Donnell, Jason Jones, and Joseph Staten jokingly referred to the monitor of Installation 01 as "7 Broken iPod" and the monitor of Installation 07 as "49 Fucking Lightbulb".
- A monitor Avatar prop is unlockable on Xbox LIVE upon downloading Halo Waypoint..
- In Forge, one can turn into a monitor by pressing up on the D-Pad to edit the map. However, unlike fought Monitors like 343 Guilty Spark, they have the same shield strength as normal players and no attacking options other than ramming or dropping large objects on hostiles.
- In Halo: Reach Forge, a monitor will bleed if shot with its shields down.
2401 Penitent Tangent patrolling on the surface of Installation 05.
343 Guilty Spark, using his force field after turning rampant.
How a Monitor's heads-up display appears in Forge.
- ^ This behavior may also have been due to 343 Guilty Spark's rampancy. The monitor may have been oblivious to all possible options that did not follow protocol -- the Halo's activation may have been the only solution that ever even occurred to him -- and he might have been willfully ignoring other solutions.
 List of appearances
- Halo: Combat Evolved (First appearance)
- Halo: The Flood
- Halo 2
- Halo 3
- Halo 4
- Halo Legends
- Halo: Evolutions - Essential Tales of the Halo Universe
- The Return (Motion comic only)
- Halo: Blood Line
- Halo: Reach (Non-canonical appearance)
- Halo: Cryptum
- Halo: Primordium
- Halo: Silentium
- ^ Halo: Cryptum, page 291
- ^ a b c Halo: Cryptum, page 255-256
- ^ Halo 2, campaign level The Oracle
- ^ Halo: Cryptum, page ???
- ^ Halo 2, multiplayer map Backwash
- ^ Halo: Cryptum, page 297
- ^ Halo: Combat Evolved
- ^ a b c Halo: Blood Line
- ^ Halo: Combat Evolved, campaign level 343 Guilty Spark (04-343 Guilty Spark:"Greetings. I am the monitor of Installation 04. I am 343 Guilty Spark.")
- ^ Halo 2, campaign level Gravemind (05-2401 Penitent Tangent: "Greetings. I am 2401 Penitent Tangent. I am the monitor of Installation 05.")
- ^ Halo: Blood Line, Issue 2 (686 Ebullient Prism: "Greetings. I am the monitor of Line Installation 1-4. I am 686 Ebullient Prism.")
- ^ Halo: Combat Evolved, campaign level Two Betrayals
- ^ a b c Halo 3, campaign level The Ark
- ^ Halo: The Flood, page 238
- ^ Halo 3, campaign level Halo
- ^ a b Halo 3, campaign level Floodgate
- ^ Conversations from the Universe - 343 Guilty Spark's Log
- ^ Halo: Combat Evolved Anniversary - Terminal 4
- ^ Halo Waypoint: Halo Anniversary Terminal Video
- ^ Halo 3 Legendary Edition - Essentials, Disc Two - Halo: Combat Evolved Cinematics Commentary
- ^ Halo.bungie.org: My writeup / tour of Waypoint. (NSF56k)