Several actors from "Star Trek: The Original Series" (TOS) have appeared in subsequent Star Trek series, sometimes reprising their roles and other times portraying different characters. Below is a list of some notable actors who made such appearances: 1. **Leonard Nimoy** - Nimoy famously played Spock in TOS and reprised his role in "Star Trek: The Animated Series" (TAS), "Star Trek: The Next Generation" (TNG), and also in the rebooted Star Trek films. Specifically, in TNG, Nimoy appeared as Spock in the two-part episode "Unification." 2. **James Doohan** - Doohan played Montgomery "Scotty" Scott in TOS and TAS and reprised his role in TNG's episode "Relics." 3. **DeForest Kelley** - Kelley portrayed Dr. Leonard "Bones" McCoy in TOS and TAS. He made a cameo appearance as an aged Admiral McCoy in the pilot episode of TNG, "Encounter at Farpoint." 4. **Majel Barrett** - Barrett was Nurse Christine Chapel in TOS, the voice of the computer in several of the Star Trek series, and played Lwaxana Troi in TNG and "Star Trek: Deep Space Nine" (DS9). 5. **George Takei** - Takei played Hikaru Sulu in TOS and TAS, and he reprised his role in the "Star Trek: Voyager" (VOY) episode "Flashback." 6. **Walter Koenig** - Koenig portrayed Pavel Chekov in TOS and TAS. He reprised his role in an episode of VOY, "Flashback," alongside George Takei. 7. **Nichelle Nichols** - Nichols, who played Nyota Uhura in TOS and TAS, did not reprise her role on other Star Trek series, but she did contribute her voice to "Star Trek: The Animated Series." 8. **William Shatner** - Shatner played Captain James T. Kirk in TOS and TAS and reprised his role in the movies. He did not appear as Kirk in later Star Trek series but did direct and appear in several of the TOS-era Star Trek films. Apart from these main cast members, several other actors who had smaller or guest-starring roles in TOS appeared in different roles in later series: 1. **Mark Lenard** - He is best known for playing Sarek, Spock's father, in the original series, but he also portrayed a Romulan commander in the TOS episode "Balance of Terror" and a Klingon captain in "Star Trek: The Motion Picture." 2. **Jeffrey Hunter** - Hunter played Captain Christopher Pike in the original TOS pilot "The Cage," and footage from this pilot was used in the TOS episodes "The Menagerie" Parts I and II. The character of Pike was later revisited in series like "Star Trek: Discovery," played by a different actor, Anson Mount. 3. **Diana Muldaur** - Muldaur appeared in two different roles in TOS ("Return to Tomorrow" and "Is There in Truth No Beauty?") and later played Dr. Katherine Pulaski in the second season of TNG. There may be additional instances not covered here, but this provides an overview of some of the most notable crossover appearances between "Star Trek: The Original Series" and other shows in the franchise. The enduring appeal of these actors and characters has helped maintain a sense of continuity and shared universe within the Star Trek franchise.