Post-apocalyptic worlds collided Sunday night when Lennie James’ character Morgan Jones left the world of The Walking Dead to enter the story of its sister series, Fear the Walking Dead, giving viewers of AMC’s zombie series the fan-satisfying crossover-episode experience.

So far, critics seem to approve: Fear the Walking Dead’s season 4 premiere, “What’s Your Story,” has a Tomatometer score of 100% on 12 reviews.

It’s hard to say how the concept of interweaving stories or characters from one project to another originated. The Paley Center for Media curator David Bushman argues that literature has used the trope for centuries, noting Mark Twain’s decision to make Tom Sawyer appear in Adventures of Huckleberry Finn. More recently, Film Twitter lost its collective mind in March over the assertion that Marvel’s Avengers: Infinity War may be the most ambitious crossover event in history and cited numerous other examples that are no doubt better because Film Twitter said so.

Whatever the origin, crossover plot tactics are extremely lucrative for the entertainment industry. So much so, in fact, that titles like Netflix’s Marvel superhero series — Daredevil, Jessica Jones, Luke Cage, and Iron Fist — used character crossovers as primary plot tactics to remind us that they operate in a larger story together even before The Defenders (pictured above) officially united them.

“Crossovers are a pain in the a–. Everybody hates doing them,” uber producer Dick Wolf told journalists at the winter 2017 Television Critics Association press tour, referring to the scheduling headaches that ensued to facilitate overlapping stories on his Law & Order and Chicago franchises. He added, however, that they’re “like ratings crack, and it’s a bad addiction because you don’t want to overuse it.”

That said, TV crossovers are unlikely to be going away any time soon. Paley Center’s Bushman says the consolidation of media companies that allows a larger studio to gobble up smaller ones and their media libraries makes it easier for events like Supernatural’s recent Scooby Doo episode on The CW. They’re also expected occurrences on the many series that fit into the CW’s CW’s Arrowverse programming and are signals to loyal fans of one ratings juggernaut that a new series in that orbit may be worth watching (see ABC’s introduction of Station 19 via Grey’s Anatomy).

But just because crossovers get good ratings doesn’t mean they get positive reviews. We consider some of the more noteworthy events in recent crossover history.

1. Supernatural–Scooby Doo

Zoinks! Supernatural brothers Sam (Jared Padalecki) and Dean Winchester (Jensen Ackles) entered the world of Scooby Doo in this much-hyped animated episode in the long-running show’s 13th season. Titled “Scoobynatural,” the episode also starred Matthew Lillard, Kate Micucci, Frank Welker, and Grey Griffin as the voices of the Scooby Gang (Lillard featured in the Scooby Doo movies). It was a ratings win and pretty much every critic ate it up like a crime-solving Great Dane at snack time.

2. The Simpsons–24

In a feat of Fox synergy, the cast of 24 leant their voices to a Simpsons parody of their high-stakes world when the pressure’s on to stop a stink bomb from igniting at Springfield Elementary. It’s one of many, many crossover stunts the long-running animated show has pulled and is often listed as a favorite when critics consider the show’s knack for both parody and casting guest talent. It also won that year’s Annie Award for best writing in an animated television production.

3. The X-Files–COPS

As weird as a crossover of the sci-fi The X-Files and crime-focused reality series COPS sounds, keep in mind that both versions are all about finding the truth that’s out there. Shot in real time and recorded on videotape, writer Vince Gilligan and director Michael Watkins’ homage to the popular police series follows Fox Mulder (David Duchovny) and Dana Scully (Gillian Anderson), as they investigate a mysterious incident in Los Angeles with the hopes that a live TV crew might garner them more public support for their research into the paranormal. The episode is almost always on critics’ lists of best X-Files episodes ever.

4. Supergirl–The Flash musical crossover

Two TV tropes for the price of one: These two episodes of the fan-favorite series Supergirl and The Flash didn’t just have overlapping story lines, there were also song-and-dance numbers (a respected feat given that both Melissa Benoist and Grant Gustin, who play the eponymous superheroes in question – as well as many others on their shows – are triple threats). The duo’s fellow Glee alum, Darren Criss, also guest stars as the Music Meister, a sort of Pied Piper-like villain who manipulates others through song. Empire magazine’s review gave it (almost) equal marks to another beloved musical episode, Buffy the Vampire Slayer’s “Once More with Feeling.”

5. CSI universe 3-way crossover

CSI: Crime Scene Investigation Season 9, Episode 5: “Leave Out All the Rest”

This crossover synergy from 2009 marked the first time in the franchise’s history that the trio of shows came together for one overarching story. Billed as “Three Cities, Three CSI Teams, One Big Mystery,” the events took place during a week of November sweeps. It followed Laurence Fishburne’s Raymond Langston as his hunt for a missing girl allows him to uncover a black market human trafficking operation. The Entertainment Weekly review argued that the final episode – which happened to be set at CSI numero uno – was probably the best, but that Fishburne himself was not necessarily that much fun to watch.

6. The Vampire Diaries–The Originals

When you’re immortal, you’ve got plenty of time to travel – particularly when you’re hiding out from a vampire hunter, as The Vampire Diaries’ Stefan Salvatore (Paul Wesley) did in this crossover episode with The Originals. While it wasn’t uncommon for characters on these shows to swap places – despite what its name suggests, The Originals was a spin-off of TVD – but fans liked that one of the Salvatore brothers made the trip to the Big Easy. Reviewers also liked that the story lines intersected in a natural way that didn’t feel like an obvious promotional ploy.

7. Scandal–How to Get Away With Murder

It was about time Scandal’s Olivia Pope and How to Get Away with Murder’s Annalise Keating got to share some screen time. During Scandal’s final season, actors Kerry Washington and Viola Davis appeared in the same episode for a one night Shondaland extravaganza. Audiences paid attention via a noticeable ratings boost and critics mostly embraced the stunt. The A.V. Club called it “fanbait,” but a “well-crafted” one.

8. The Flash–Supergirl–Legends of Tomorrow–Arrow 4-way crossover

This ambitious four-way crossover event has the heroes of the Arrowverse banding together to stop an alien invasion, marking the first time characters from all four of that world’s live-action shows would join forces at the same time. The spectacle, for which all shows but Supergirl were coyly titled “Invasion!,” was such a commercial and critical success that they repeated their efforts the following season.

9. Law & Order crossovers

While Richard Belzer’s John Munch has popped up in several series (even in Arrested Development!), the Law & Order franchise has long made use of crossover episodes with intersecting story lines over various shows. The first time it happened with Law & Order: SVU was in 2000, which was that show’s freshman season. The gamble that audiences would double-down on gritty crime dramas paid off, with The Washington Post saying the stunt packed “twice the punch.”

10. Chicago crossovers

Chicago Fire, Season 5, Episode 15: Deathtrap

Chicago P.D. Season 4, Episode 16: Emotional Proximity

Chicago Justice, Season 1, Episode 1: Fake

Another Dick Wolf franchise, the Chicago series shares Law & Order’s penchant for showy crossover episodes. There have been two instances of crossovers involving just Chicago shows, the most recent one happening in 2017 when the franchise welcomed Chicago Justice to the family (and was thus the reason why Wolf was grumbling earlier). Not only did the stories cross over among the Chicago Justice, Fire, and P.D. series, but some of the Chicago Med characters also came along for the ride.

11. Station 19 premiere–Grey’s Anatomy crossover

This episode, which set up the story for Grey’s Anatomy spinoff Station 19, reminded audiences that they now had the option to watch hot firefighters learn how to save a life. It also welcomed actress Jaina Lee Ortiz’s Andy Herrera into the family of powerhouse Shondaland women. According to an interview with creator Stacy McKee, there are still chances for more crossovers to come.

12. Parenthood–About a Boy

About a Boy, Season 1, Episode 7: About a Poker Night

Parenthood, Season 5, Episode 13: Jump Ball

Of course Parenthood’s Crosby (Dax Shepard) and About a Boy’s Will (David Walton) are poker buddies. Not only did they both have man-child tendencies and live in the same city, but they’re also fictional characters on two shows developed by Jason Katims. Crosby was a natural fit to introduce Will to Parenthood’s Kleenex-clutching audience. (Incidentally, this is not the only time Katims has made his series’ worlds intersect. Some cast members from Friday Night Lights also appeared in a Parenthood web series).

13. Brooklyn Nine-Nine–New Girl

Zooey Deschanel’s Los Angeles lady, Jess Day, got more than she bargained for on her New York vacation when Detective Jake Peralta (Andy Samberg) needed her help with a case. This attempt at comedy synergy, however, was not as open-shut. Slate ran a lengthy take-down on what was described as a “missed opportunities” for both shows and IndieWire called it an “ill-fated pairing.”

14. Bones–Sleepy Hollow

Bones’ Brennan (Emily Deschanel) and Booth (David Boreanaz) help Sleepy Hollow’s Ichabod (Tom Mison) and Abbie (Nicole Beharie) solve a case of historical significance in this Halloween-themed crossover. It drew decent ratings, but critics were mixed. TV Line argued that, for a scenario involving particularly eccentric circumstances and people, the event was “just not that weird.”

15. The Simpsons–Family Guy

Fox animated series The Simpsons and Family Guy each have strong fan bases — so devoted are they that they sometimes turn on each other to argue similarities in the shows’ premises or episodes. So it seemed appropriate that the two would join forces for a two-part episode of the latter to prove that everything is derivative — with a special surprise guest making the final ruling. Vulture called it an “anti-classic” while The Telegraph argued that it suffered from too much forced humor.

Did you watch The Walking Dead–Fear the Walking Dead crossover? What did you think? Tell us in the comments.