Minefield
Not so much an appearance by the Romulans as a physical presence in the form of their deadly orbital mines and specifically one that attaches itself to the hull of Enterprise. When Reed tries to diffuse it, his leg becomes impaled by one of its spikes. This episode, if you will pardon the pun, gets Starfleet started off on the wrong foot with the Romulans and is merely a harbinger of things to come. This is where it all begins, taking the Star Trek saga chronologically.

Balance of Terror
In terms of our audience perspective, this is the one that started it all. The first of many great performances by Mark Lenard (who would also don pointed ears to play Sarek), this episode is one of the finest Star Trek episodes of any series, giving fans not only a great cat-and-mouse pursuit story, but a gritty, harsh take on the necessities and costs of war in general. The regret Kirk feels at the Romulan Commander's death is shared by the audience, a true feat of complex storytelling in a genre too often filled with two-dimensional villains whose deaths are cheered. Given today's political climate, it is as relevant as ever.

The Enterprise Incident
The Cold War metaphors abound in this clever, twisting episode where all is not what it seems. In the end, Kirk, Spock and the Federation are the ones guilty of deception, but the ends (achieving parity with the Romulans and their cloaking technology) justify the means (incursion into Romulan space and espionage). While normally we'd expect such tactics from the shifty Romulans instead of the squeaky-clean Federation, this episode does give us a further glimpse of the fact that Romulans are not all that different from Humans. One almost feels sorry for the Romulan Commander, romantically betrayed by Spock. But all's fair in love and war, no?

The Neutral Zone
This episode marked the return of the Romulans, updating them for the TNG era. A little craftier, a little more sinister, these were the next generation of Romulans. While budgetary limitations forced the show to use Klingon ships in their last TOS appearance ("The Enterprise Incident"), the classic Federation foes re-emerged with the new-and-improved D'deridex-Class Warbird, a fierce, daunting ship that inspires a shiver when it decloaks. With the ominous declaration of "We are back" after an absence of decades, this confrontation with the Enterprise-D set the stage for adventures and conflicts to come. And yes, much like the Klingons, they also had a bit of a makeover facially.

The Enemy
Romulans had always been portrayed as militant, quick to defend what they perceive as their own territory, and referred to as treacherous. In this episode, we get to see them in full deception mode, dissembling to cover their tracks and telling lies within lies and omitting truths as it suits them. While the theme of the story itself reflects how two enemies can unite when faced with adversity, the deep-seated culture amongst Romulans for deceit and trickery is apparent.

The Defector
Upping the ante from their last appearance, where they had to lie to cover up a mishap, this episode shows the Romulans engaged in all-out subterfuge intended to provoke a war by using one of their own officers as a pawn in a high-stakes game. Despite the Romulan deceit, Jarok's honor displays evidence that the Romulans are not all tricksters, but a society under the control of the likes of Tomalak.

Data's Day
Not only does this episode show that the Romulans have managed to have a high-placed, high-profile spy living amongst Vulcans, it's one where our heroes don't come out on top in the end. Unlike most episodic television, the Romulan "victory" gives the series that taste of reality — good and honorable behavior doesn't necessarily ensure the most positive outcome.

Redemption, Part II
The introduction of Sela and her backstory gives us a further taste of Romulan society. After the events seen in "Yesterday's Enterprise," Sela's father found Tasha Yar desirable, and used his clout to ensure she'd avoid execution, but at a price. Back in the present, the Romulan machinations to influence the outcome of the Klingon Civil War demonstrate their cunning, and when they're found out, their ruthless desertion of the House of Duras reveal a cold, pragmatic wisdom.

Unification, Part II
During this episode we not only get to see Spock for the first time since "Star Trek VI: The Undiscovered Country," but we also get our first glimpse of Romulus itself. This episode also gave a good example of how a so-called "enemy" is not an entire race, but the political structure in power at any given time. Unlike an unredeemable foe such as the Borg, there is hope for the Romulans, and that hope is worth pursuing.

Face of the Enemy
This adventure not only gave Deanna Troi a chance to get in on the action normally reserved for other characters, but it also gave fans a look at some true Romulan heroes willing to sacrifice their lives for the cause of peace. As unlikely as a Klingon/Federation alliance may have seemed to TOS fans, this episode shows that someday, perhaps years away, the Romulans will cease to be the threat to the Federation their current leadership provides.

Eye of the Needle
A very rare appearance of Romulans in Star Trek: Voyager, this episode shows us a Romulan scientist from an earlier era. While this episode sheds little light on the Romulans as a people or as a political force, it shows even Romulan scientists are beholden to the same ideals of objective truth and reason, regardless of politics.

The Die is Cast
While giving further insight into the workings of the secretive Tal Shiar, this episode also shows that the Romulans are nothing if not always seeking to be ahead of the game — proactive to potential enemies of the Romulan Star Empire; seeking to destroy the Dominion threat before actual hostilities are declared. The irony is that the Dominion is way ahead of the Tal Shiar, having infiltrated the covert agency, and the Romulan/Cardassian plot backfires with immense loss of life and ships.

Star Trek Nemesis
Picard and his crew face a new threat from the Romulan Star Empire but this time the threat comes in the form of a cloned human, Shinzon, who offers his services and those of his Reman followers to the Romulans in an effort to make the empire stronger. When the Romulans refuse Shinzon's overtures, he takes matters into his own hands and attempts to destroy the person he was cloned from, Captain Picard, along the U.S.S. Enterprise-E and the Federation itself. The ultimate defeat of Shinzon leads to peace overtures between the two superpowers, with Captain Riker of the U.S.S. Titan in charge of commencing talks. It is here that the Romulan trail ends. For now.