Romulan technology was often cited as one of the reasons why the Romulan Star Empire was regarded as such a dangerous foe in times of war. The Romulans were known to have developed a multitude of different technologies that are either the first of their kind or unique to the Romulan Star Empire. Some Romulan technology was regarded highly enough that the Federation even resorted to stealing an example of the cloaking device in 2266, although the Treaty of Algeron prevented them from using it. (TOS: "The Enterprise Incident")
Stealth and Holographics
The Cloaking Device
A Romulan cloaking device
The Romulans were the first known race to develop the cloaking device stealth technology, first observed on Romulan mines and the Romulan Bird-of-Prey in 2152. (ENT: "Minefield")
While the Suliban were known to use cloaking devices by 2151, this technology was provided by a mysterious benefactor from the 29th century and was not developed by their race. (ENT: "Broken Bow", "Shockwave", "Future Tense")
During the brief Romulan-Klingon Alliance in 2268, the Klingon Empire provided the Romulans with an unknown number of the Template:ShipClass battle cruiser in order to acquire Romulan cloaking technology. (TOS: "The Enterprise Incident")
The Romulans and the Federation long engaged in a tech war of cloak and cloak detection. In 2152, a quantum beacon proved an effective counter measure for detecting less advanced cloaks such as those found on Romulan mines, but not those on early Romulan birds-of-prey. In 2368, the Federation developed the tachyon detection grid, a technique that allowed for the detection of Romulan vessels of the day. (TNG: "Redemption II")
By 2379, the Star Empire had perfected the cloaking technology. The cloak of the Reman Warbird Scimitar produced no tachyon emissions and left no residual antiproton traces for detection. It is possible the IRW Valdore and her sister ship incorporated similar systems as they were seemingly undetectable even to the Scimitar when cloaked. The Scimitar was the only vessel in the Star Empire known to have overcome the long standing problem of not being able to raise shields or fire weapons when the cloaking device is active. The only known way to counter these latest cloaks involved the use of telepathic races such as Betazoids to detect an enemy crew by sensing their thoughts. (Star Trek Nemesis)
Multispectral emitters
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Holographic projector technology equipped on the Romulan drone-ship, a multispectral emitter was used as a camouflage system. The emitters could produce a holographic image that could mimic the appearance of almost any ship. When activated, the emitters made it nearly impossible for an enemy to acquire a target lock on the ship's primary systems, such as weapons, although the emitters themselves could be targeted. (ENT: "Babel One")
Tri-phasic emitters
Equipped on board the Romulan drone-ship, tri-phasic emitters could simulate different types of weapons used by various other races, including both disruptor and phaser based technologies. (ENT: "Babel One")
Site to site holographic emitters
Shinzon hologram walks through Captain Picard's desk
This technology could capture live sounds and images of a person in one location and project those images to another location. Images and sound from that second location could be detected and transmitted back to the first location, making two way interaction possible.
The first known use of these emitters was in 2379 when Shinzon of Remus requested the USS Enterprise-E to stand down its defences and surrender Captain Jean-Luc Picard to the Scimitar. The source of the hologram once projected was untraceable. (Star Trek Nemesis)
An image could be sent from a ship that was under cloak and the technology was known to be able to penetrate the deflector shields of a starship. The emitters were only observed when used between ships in relatively close proximity. When used by Shinzon, his hologram was seemingly able to sit in chair but also pass through a table. (Star Trek Nemesis)
Weapons
Romulan weapons were described by Spock in 2266 as being superior to those of the Federation. (TOS: "Balance of Terror")
Disruptors
Romulan disruptor pistol
The Romulans were one of three races along with the Klingons and the Breen who were known to use the Type-3 disruptor. (Star Trek Generations)
When fired, a Romulan disruptor left behind residual antiproton particles that remained for several hours. (TNG: "Face of the Enemy")
Plasma Torpedoes
A Plasma torpedo being fired
First known to have been used by the Romulans in 2266, plasma torpedoes were an extremely powerful yet energy intensive weapon system. This type of high-energy plasma weapon was first encountered by Starfleet when a Romulan Bird-of-Prey destroyed several Federation outposts near the Romulan Neutral Zone.
Upon detonation, the torpedo would unleash an enveloping energy plasma, forcing an implosion in the target. Even Castrodinium, the hardest material known to Federation, could not withstand the effect. One torpedo was capable of devastating outposts sunk a mile deep on asteroids of near solid iron, even when protected by deflector shields. A second torpedo could then turn these asteroids to dust. The weapon was capable of following a vessel going to warp speed. The range of the torpedoes was extreme, but limited. (TOS: "Balance of Terror")
The only other races to have used plasma torpedoes are the Kazon and the Cardassians, but the first documented usage of plasma torpedoes by these races comes more than a century after the Romulans, in the early 2370s. (VOY: "Projections") (DS9: "Tears of the Prophets")
Thalaron generator
Part of a Thalaron generator
This super weapon was capable of creating a thalaron radiation field large enough to encompass a ship or an entire planet.
Previously thought to be simply theoretical by Federation scientists, research into thalaron radiation was banned by the Federation sometime prior to 2379 due to its biogenic properties. Thalaron radiation consumed organic material at the subatomic level, causing instant and complete necrosis in every cell it irradiated. A matter of seconds after exposure, the organism degenerated into an ash-like material. Even a minuscule amount of thalaron radiation could kill every living being aboard a starship. (Star Trek Nemesis)
Defences
Shields
Standard Romulan deflector shields were believed to be at least on par with those of the Federation. However, the most advanced example of shield technology in the Romulan Star Empire was present on the Scimitar', which was equipped with both primary and secondary shields. The ship endured at least 32 phaser hits, 20 disruptor strikes and 20 torpedo impacts in a single engagement at the Battle of the Bassen Rift, exhausting the weapons compliment of the Enterprise-E, and was still able to maintain 70% shield strength. (Star Trek Nemesis)
Ship self-repair technology
The Romulans were known to be able to equip their ships with a self-repair technology, principally used on the Romulan drone-ship in 2154. (ENT: "United") It is unknown if this technology was incorporated as standard in other Romulan ships, such as the Warbird model that the drone-ships were modified from.
Remote ship control
An Aenar piloting a Romulan drone-ship
In 2154 the Romulans possessed the technology to facilitate the control of ships by remote telepresence. The only race known to possess the telepathic ability to actually control a ship in such a way were the Aenar, a subspecies of the Andorians. As the Aenar were strictly pacifist, one (Gareb) had to be kidnapped and drugged for the technology to be put to use. (ENT: "Babel One")
Propulsion
Artificial quantum singularity reactor
An artificial quantum singularity in a Romulan engine core
In the 24th century, an artificial quantum singularity would serve as the primary power source for large Romulan spacecraft, such as the Template:ShipClass Warbird. Once activated, the singularity core cannot be shut down. (TNG: "Face of the Enemy", "Timescape"; DS9: "Visionary")
A relay station network of ancient origin, claimed by the Hirogen, was the only other known example of a race using quantum singularities as a power source. (VOY: "Hunters")