The 1990s were a golden age for Star Trek as the popularity of Star Trek: The Next Generation spawned two spinoff series which regularly crossed over, creating an interconnected fictional universe that was ahead of its time. Star Trek: Deep Space Nine was the first of these spinoffs, premiering in 1993 with an episode that featured Patrick Stewart and established Colm Meaney's Chief O'Brien. The DS9 pilot continued a tradition established by the TNG premiere, which featured a cameo by DeForest Kelley as Dr. Leonard "Bones" McCoy, a touching way of ing on the torch to the next generation.
When Star Trek: Voyager premiered in 1995, its first mission launched from Deep Space Nine, meaning that characters from both shows briefly interacted. There were occasionally attempts made at more ambitious crossovers, like a Star Trek: Deep Space Nine season finale that would feature the USS Enterprise-D defending the station from invading Cardassians. However, these proved too difficult to realize, so instead the 1990s Trek shows had an extensive list of smaller crossover episodes that presented the Star Trek universe as a sprawling fictional world for the first time ever.
21 DS9: Emissary (Jean-Luc Picard)
As the first of Star Trek: The Next Generation's spinoffs, it was necessary for a character like Captain Jean-Luc Picard (Patrick Stewart) to appear in the Star Trek: Deep Space Nine to on the torch. The scenes between Picard and Commander Benjamin Sisko (Avery Brooks) were electric and established how different a show DS9 would be. Sisko had problems with Jean-Luc Picard, and it was very clear that, as Bajor's Emissary, Sisko's loyalty wouldn't always be to the Federation. This was in sharp contrast to Picard's unwavering belief in the Prime Directive and the mission of the Federation, making his cameo the perfect way to lay the groundwork for the show that DS9 would become.
20 DS9: Past Prologue (Lursa & B’Etor)
Star Trek: TNG's villainous Duras sisters Lursa (Barbara March) and B'Etor (Gwyneth Walsh) appeared in the very next episode of DS9. This was presumably to further emphasize the connections between both shows. In "Past Prologue", the Duras sisters tried to play Bajoran terrorist Tahna Los (Jeffrey Nordling) and the Cardassians against each other. They sold Tahna the powerful element bilitrium to use in a weapon with which he planned to destroy the wormhole. Unbeknownst to Tahna, the Duras sisters were also feeding information to Garak (Andrew J. Robinson) with the hope of profiting from both sides of the Cardassian-Bajoran conflict.
19 DS9: Q-Less (Q and Vash)
Sisko's one and only confrontation with Q (John de Lancie) further established his differences from Picard. Nobody thought to just punch the trickster god in the face until Commander Benjamin Sisko. It was the final confirmation to audiences that Sisko was "not Picard", and ensured that Q and Sisko never did battle again. In "Q-Less", the cosmic trickster visited DS9 to attempt a reunion with Picard's ex-lover Vash (Jennifer Hetrick), but he failed to win her back, losing the archeologist to her new business partner, Quark (Armin Shimerman).
18 DS9: The Forsaken, Fascination, and The Muse (Lwaxana Troi)
Lwaxana Troi (Majel Barrett) the mother of TNG's Counselor Deanna Troi (Marina Sirtis) made three appearances in DS9. As a Betazoid Ambassador, she first visited the station as part of a fact-finding mission to the Gamma Quadrant. Lwaxana formed an immediate attraction to Constable Odo (Rene Auberjonois), and the pair had a close friendship throughout her DS9 episodes. Odo even married Lwaxana Troi to protect her from being separated from her unborn child. Lwaxana also helped Odo realize his feelings for Major Kira Nerys (Nana Visitor), which he would eventually act upon in DS9 season 6.
17 DS9: Multiple Episodes (Gowron)
Klingon Chancellor Gowron (Robert O'Reilly) appeared in multiple DS9 episodes, featuring most prominently from season 4 onwards as the Dominion manipulated the Klingon Empire into declaring war against Cardassia. The loyalty between Commander Worf (Michael Dorn) and Gowron would be tested throughout DS9, culminating in a ritualistic duel to the death in DS9's final season. Gowron's first crossover appearance was in a much lighter episode, season 3's Klingon wedding comedy "The House of Quark". The episode saw Quark roped into a scheme by Klingon widow Grilka (Mary Kay Adams) to keep control of her House following the death of her drunken husband, bringing the Ferengi barman face to face with Gowron.
16 DS9: Defiant (Thomas Riker)
In season 3, "Defiant" was one of DS9's biggest crossovers up to that point, featuring the appearance of Jonathan Frakes as Commander William T. Riker. Except, he wasn't playing Will, he was playing Riker's transporter clone Thomas. The episode revealed that Thomas Riker had defected to the Maquis and stole the USS Defiant to expose the existence of secret Cardassian military installations. By the end of the episode, Thomas was placed in Cardassian custody. Jonathan Frakes and the DS9 writers' room had wanted to bring the character back at some point, but the opportunity never materialized.
15 DS9: Sons of Mogh (Kurn)
Worf's brother Kurn (Tony Todd) made a single crossover appearance in "Sons of Mogh", in which he requested that Worf kill him to restore his honor. After Worf had opposed the war against the Cardassians, the House of Mogh was in disgrace, and Kurn could no longer live with the shame. Although Worf initially attempted to carry out the ritual, he was stopped by Odo and Lt. Jadzia Dax (Terry Farrell). Realizing that giving Kurn a new purpose as a security officer on DS9 still didn't solve the problem, Worf had his brother surgically altered and his memories erased. Kurn left DS9 to live a new life as Rodek, tragically leaving Worf more alone than before.
14 DS9: Sons And Daughters, You Are Cordially Invited (Alexander Rozhenko)
Worf's son Alexander Rozhenko (Marc Worden) is featured in two episodes of Star Trek: DS9, including his attendance at Worf and Dax's wedding in "You Are Cordially Invited". Alexander first returned in the season 6 episode "Sons and Daughters" when Worf was surprised to find his son serving aboard the IKS Rotarran. Alexander's clumsiness was a source of frustration and embarrassment to Worf, however, he eventually realized that he must let his son go his own way. After attending his father's wedding, Alexander transferred to the IKS Ya'Vang for the remainder of the Dominion War.
13 DS9: Doctor Bashir, I Presume? (Dr. Lewis Zimmerman)
When Dr. Julian Bashir was selected to be the model for the next generation of Emergency Medical Hologram, it was only natural that Dr. Lewis Zimmerman (Robert Picardo), creator of Star Trek: Voyager's EMH would visit Deep Space Nine. Zimmerman was certainly a genius, but he also had a weakness for beautiful women, gambling, and drinking. This was in sharp contrast to his creation, who had more highbrow interests like opera and literature.
12 DS9: Through The Looking Glass (Mirror Tuvok)
Star Trek: Voyager never visited the Mirror Universe, so Tim Russ holds the distinction of being the only cast member to play his darkest timeline counterpart. "Through the Looking Glass" aired in April 1995, only three months after Voyager had begun, so it was a neat nod to DS9's newest franchise stablemate. In the episode, Sisko was sent to the Mirror Universe to convince Jennifer Sisko (Felecia M. Bell) to the Terran Rebellion, of which Mirror Tuvok was a member.