Although a span of a century separated the two shows within Star Trek canon, several characters from Star Trek: The Original Series appeared in Star Trek: The Next Generation. Set in the 2360s, The Next Generation was in an entirely different era than The Original Series, which took place in the 2260s, and yet notable figures reprised their roles in one way or another. The production of the Star Trek movies ran concurrently with The Next Generation, which made crossovers easy for the celebrated cast of the classic show. Though they mostly played small parts, the Original Series characters appeared in some of the best episodes of The Next Generation​​​​​.

Starting as early as TNG season 1, episode 1, "Encounter at Farpoint," figures from Star Trek's past were on hand to christen the new series as it began its seven-year run on the air. Many early episodes of TNG were based on TOS stories, and it was clear that the successor series could never fully get out from under the shadow of the original, though that wasn't a bad thing. Appearances from TOS characters not only gave the series continuity by firmly establishing it in the canon, but it also gave casual fans something familiar to look for as they experienced new adventures on the USS Enterprise-D.

RELATED: TNG Invented The Star Trek Series Finale

Dr. Leonard McCoy - "Encounter At Farpoint" (S1, E1)

Doctor McCoy and Data walk down the hall in Star Trek TNG

Playing the smallest role of any Star Trek: The Original Series alumnus who returned on Star Trek: The Next Generation, Dr. Leonard McCoy appeared for only a moment at the end of the pilot episode, "Encounter at Farpoint," and he gave Data sage words of advice. Even at the age of 137 in the year 2364, the old country doctor still had a fond love for the Enterprise and had lost none of the character traits from Star Trek that made him instantly recognizable. Speaking sentimentally about the ship, McCoy told Data to "treat her like a lady," reminding the puzzled android that she would always be good to him.

McCoy was the only human character from The Original Series to return on The Next Generation under natural means, and his extremely aged appearance did an excellent job of establishing just how much time had passed between the two shows. Even so, McCoy's cameo also stood as a testament to the continuity of the Star Trek universe, and that even in the future of the future, things weren't all that different after all. McCoy's gruff demeanor was exactly as it had been a century before, even down to his distrust of transporters that hadn't abated in his near century-and-a-half of life.

Sarek - "Sarek" (S3, E23) & "Unification Part 1" (S5, E7)

Sarek speaks while standing on the transporter pad from Sarek

Spock's father, Sarek, first re-appeared in Star Trek: The Next Generation, season 3, episode 23, "Sarek," when he was invited aboard the Enterprise to conclude peace negotiations but was struck with debilitating effects from his Bendii syndrome. By the time of The Next Generation's third season, Sarek was already 200 years old, which was an advanced age even for Vulcans. Despite his illness, Sarek insisted on concluding the talks, an indication of his true dedication to peace and logic.

Sarek also appeared in TNG, season 5, episode 7, "Unification Part 1," but his age had further weakened him, and he merely wished for Picard to help find his son Spock. Sarek's time on TNG was unique among TOS characters because he was the least developed before he turned back up again in the 24th century. Sarek had previously appeared in one of the best Star Trek Vulcan episodes, TOS season 2, episode 10, "Journey To Babel," and even his TOS movie appearances were scant. His two-episode stint on TNG allowed for more of his complex character to shine through, even if it was showcased in his sad last days.

RELATED: Every Actor Who Has Played Spock On Star Trek

Spock - "Unification Part 1 & 2" (S5, E7 & 8)

Spock looks on in Star Trek TNG

When he served as first officer aboard the USS Enterprise under Kirk in the 2260s, Spock was actually quite young for a Vulcan and merely middle-aged when he appeared in Star Trek: The Next Generation. The 138-year-old Vulcan was the subject of season 5, episodes 7 and 8, "Unification," and his mysterious task within Romulan space made for a scintillating story. Spock was serving as a member of Starfleet's diplomatic corps when he went rogue and attempted to broker peace with the Romulans. Spock's Romulan mission was explained in TOS, but it finally came to fruition in TNG, and the cagey Vulcan had obviously aged since he was last seen.

Spock's dedication to peace was bound by his Vulcan logic, and he would stop at nothing to end a lengthy conflict between the Romulans and Vulcans. So much more than a mere nostalgia trip, Spock's appearance in TNG was a glimpse into his life after the TOS movies and showed what the character had accomplished after that chapter of his life ended. Spock had learned a thing or two from his previous captain and instituted his own form of "cowboy diplomacy" even if it went against the wishes of his superiors. Not only had Spock aged, but he had also grown as a character in the intervening years.

Montgomery Scott - "Relics" (S6, E4)

Scotty and Picard seen on the original enterprise bridge in Relics

Using an old trick to save the Enterprise, Scotty appeared in a truly memorable episode of Star Trek: The Next Generation that finally gave the character a proper sendoff. The Next Generation season 6, episode 4, "Relics," found the Enterprise investigating a Dyson Sphere where they discovered Scotty trapped in the transporters' pattern buffer. Lost for over 75 years, Scotty was brought aboard the newer Enterprise and treated as an honored guest, but it was obvious that the legendary engineer wanted to make himself useful. Unfamiliar with the 24th century's new technology, Scotty found himself, as the episode's title suggested, a relic.

Besides being a throwback episode that gave viewers more of the TOS character that they loved, "Relics" also had a lot to say about the place of older people in society. Despite the 75 years of missing time that he experienced, Scotty was no older than he was when he disappeared, and technology left him behind. Scotty stole the show in "Relics" as a guest star, and it drew parallels between the way older people were treated in the 20th century with the way they were still treated in the 24th. "Relics" was truly one of the best Star Trek episodes ever, and this was due in large part to Scotty.

MORE: Why Star Trek TNG Ended After Season 7 (Was It Canceled?)