I hope you enjoy these crossovers. I included official, unofficial, and disputed crossovers. There are several sites which helped me in putting this together: Thomas Holbrook's site Crossovers and Spin Offs and Keith Gow's and Ash Crowe's site The Tommy Westphall Universe. Recently I came across Robert E. Wronski Jr.'s Television Crossover Universe which has given me a few insights. Without those resources, I would not be as far along as I am now. I am very thankful for their work even thought there are a few places where I disagree with Holbrook, Gow, Crowe, or Wronski.
I have been working on verifying and adding to or severing Westphall crossovers.
There are some crossover continuities which are not covered here because they are well covered elsewhere such as Stephen King's and Quentin Tarantino's separate universes and the Pixar Theory. If my research connects a continuity with the ones currently here, they will be included then.
I will not mention all of the character crossovers between series in a franchise. So, I will not break down all of the characters who crossed between the various Stak Trek, Law & Order, or Doctor Who franchises. I may mention characters who crossed over between the franchises within the Wolf universe.
The crossovers are mostly in chronological order by the episodes' air dates.
I have many personal rules I use to determine if two series crossed over or not. I am pretty strict with them, however I have made a few exceptions.
TV series not considered
I do not consider crossovers with the following types of films, TV series, and episodes...
Animated series do not form crossovers with live action series unless the animated series is considered canon with a live action series which already crossed over.
Television series without a continuing story throughout the entire run of the series
Anthology series such as The Twilight Zone, The Outer Limits, and Love, American Style
Sketch comedy series such as Saturday Night Live
Variety series such as The Brady Bunch Variety Hour
Reality series such as COPS.
Dream episodes (Yes, I know about Tommy Westphall from St. Elsewhere and Dr. Robert Hartley from The Bob Newhart Show dreaming Newhart.)
Non-contiguous parent or child films and television series (however they may still be noted)
People not considered
People appearing as themselves and historical figures are not considered for crossovers. When Jon Cryer, Charlie Sheen, and Angus Jones appeared on CSI: Crime Scene Investigation
Two and a Half Deaths as themselves, not their characters from Two and a Half Men, the series did not cross. Mark Twain visiting the Enterprise-D on Star Trek: The Next Generation
Time's Arrow will not create a crossover with any other series where Mark Twain is a character.
The more specific a reference, the better chance I will consider it a crossover.
When a reference is specific, a crossover did occur. When the titular Luther mentions John Munch by name and specifically mentions where he works, the SVU; a crossover occurred with Law & Order: Special Victims Unit.
When a reference is vague, a crossover did not occur. On Haven
The Trial of Audrey Parker, Agent Audrey Parker mentions an agent Agent Byron Howard trained who was chasing aliens, a crossover did not occur with The X-Files. Had Agents Fox Mulder, Dana Scully, or any other agent from The X-Files been mentioned by name, this would make a cross, however, the reference is too vague to be legitimate. I would have allowed it if any of the writers or the director had been involved with The X-Files.
Reference specificity also includes the use of names. If the names are not the same, then no crossover occured unless the name change is addressed directly on screen or by the creators of the crossover.
Big (and little) fake companies fall under this rule.
If two series only share stock footage, it does not mean they crossed over. However, if the events in the stock footage are mentioned, it just might create a crossover.
After all of my other rules are applied, there are some crossovers which would be considered too vague to count. Sometimes I come across a good reason to allow the crossover. So if the argument for the crossover is good enough, I will include it.
I feel like it
And after having a good reason, sometimes I just feel like saying two series crossed over with each other. The proof may be scant, but sometimes I just decide to add it.