Cross the Streams (also known as Crossing the Streams) is in reference to when two or more Proton Streams cross paths.
It was spoken of twice by Egon Spengler, (and performed once,) in 1984. During the Ghostbusters' first case, in the Sedgewick Hotel, Egon warns everyone to never "Cross the Streams", which refers to the proton beams from multiple Particle Throwers crossing paths during use. Egon noted "It would be bad."
Several weeks later, to defeat Gozer, Egon told everyone to cross the streams, knowing full well the risks. Seven years later, on the Thanksgiving 1991 holiday, the Ghostbusters crossed the streams to destroy Ivo Shandor.
The Real GhostbustersEdit
During the Captain Steel and Dr. Destructo incident, the Ghostbusters were mistaken as supervillains by Steel. Captain disarmed them with his heat vision and bent a steel pole around all of them. Captain picked them up and flew off with them in hand but Dr. Destructo blasted Steel making him drop the Ghostbusters. In order to save themselves from falling to their death, the Ghostbusters pointed their Particle Throwers at the water below and crossed the streams.  This created a large plume of water that cushioned their impact. Egon Spengler later admitted he didn't know it would have saved their lives or not.
In theory, crossing the streams on the Temple of Gozer's doorway took the Ghostbusters right up to the line of complete subatomic destruction. For a split second, they were connected to every reality at once via Gozer's portal. Every thought they ever had, the very core of their beings, were seeded into the subconscious of the Multiverse. The concepts and methods of the Ghostbusters were adopted by parallel versions of the team.  During one spring season, nine Ghostbusters crossed the streams on Tiamat in a battle atop Dana's current apartment. They were returned to the physical plane and both Dana Barrett and Louis Tully were restored to normal but Tiamat's presence was still felt when Blood Rain showered on them and a disembodied laugh could be heard.
During the finale of the dimensional bleed incident, the two teams of Ghostbusters successfully captured the Bronx Spook but the breach left behind did not close. Erin Gilbert realized they needed a catalyst to close the breach and start the reaction that would work on the molecular level to separate the two dimensions. She suggested crossing the streams. Ray objected and called it a terrible idea. Peter was more open to it since they survived the last few times. Egon agreed with Ray but knew there was no other choice. After they crossed the streams, Jillian Holtzmann admitted she was panicking and observed it felt like being pulled in a million directions at once. The atoms of both worlds began to scatter, mingle, and separate as they came very close to the verge of protonic reversal. For seconds, the universes were joined and everything they experienced became one timeline then the fabric of reality was shaken enough and the two dimensions were restored to normal. Egon hypothesized they may be some residual memories from the atomic co-mingling. Rebecca Gorin likened it to a living representation of Escher's Relativity.
- Ghostbusters: The Video Game (Realistic Versions)
- The Real Ghostbusters
- Extreme Ghostbusters
- Ghostbusters: The Video Game (Stylized Versions)
- IDW Comics
- In Ghostbusters: The Video Game, in the realistic versions, if the player crosses the streams with a teammate for too long, then the streams will overload, sending a pulse back along the stream and knocking both characters down.
- In the realistic version, after Museum of (Super)Natural History, the seventh message on the Firehouse answering machine inquires about crossing the streams in a humorous manner. 
- On page 19 of Ghostbusters Volume 1 Issue #3, Peter Venkman suggests they use the cross the streams strategy to save the day. 
- On page 18 of Ghostbusters Volume 1 Issue #15, Winston Zeddemore suggests crossing the streams on the Megaspook but Egon shoots down the idea on the notion there is no extradimensional portal to skew the results. 
- On page 19 of Ghostbusters Volume 2 Issue #20, Peter alludes to Crossing the Streams. 
- On page 3 of Teenage Mutant Ninja Turtles Ghostbusters Issue #3, Ray mentions to not cross the streams. 
- On page 11 of Teenage Mutant Ninja Turtles/Ghostbusters Issue #4, Winston mentions crossing the streams. 
- The subject of Ghostbusters: Deviations revolves around what if the Ghostbusters never crossed the streams on the Temple of Gozer because of the inherent catastrophic danger in doing so. Gozer even suggests they do it so he can return to his home dimension but they still refuse and cite the whole planet could be destroyed.
- On page 18 of Ghostbusters 101 #3, Jillian Holtzmann alludes to the urinating joke that came from the crossing the streams term in the first movie.
- ↑ Egon Spengler (2009). The Real Ghostbusters - "Captain Steel Saves the Day" (1987) (DVD ts. 11:33-11:35). Time Life Entertainment. Egon says: "Aim at the water and cross the streams!"
- ↑ Egon Spengler (2017). IDW Comics- "Ghostbusters 101 #3" (2017) (Comic p.18). Egon Spengler says: "It took us right up to the line of a complete subatomic destruction. And by way of Gozer's portal, for a split second, we were connected to every reality at once. Every thought we ever had, the very core of our beings... that's how they were seeded into the subconscious of the Multiverse. Our concepts, our methods... In theory, anyway. But so far, observation has only lent the idea credibility."
- ↑ Ray Stantz (2009). The Real Ghostbusters - "The Boogieman Cometh" (1986) (DVD ts. 18:12-18:17). Time Life Entertainment. Ray says: "The laws of physics are different here. It could be even worse than crossing the streams!"
- ↑ Egon Spengler (2009). Extreme Ghostbusters- Darkness at Noon, Part 2 (1997) (DVD ts. 2:55-2:59). Sony Pictures Home Entertainment. Egon says: "The important thing to remember is that you must never under any circumstances, cross the streams."
- ↑ Ghostbusters 101 Class Notes (2017). IDW Comics- "Ghostbusters 101 #4" (2017) (Comic p.23). Ghostbusters 101 Class Notes reads: "All packs have one important rule: do NOT cross the streams. This can cause a chain reaction, which may lead to total protonic reversal (or destruction at the cellular level). Crossing streams is like Russian roulette, protonic reversal may never happen. But it might."
- ↑ Male Caller; After Museum of (Super)Natural History, Firehouse 2nd Floor Answering Machine Message 7 of 11 (2009). Ghostbusters: The Video Game (Realistic Versions) - Firehouse (2009) (PC/PS3/Xbox 360). Terminal Reality. Male says: "I was just calling because I was really confused about something. Me and my buddies have been going fly fishing for years and never once had any trouble crossing the streams."
- ↑ Peter Venkman (2011). IDW Comics- "Ghostbusters Volume 1 Issue #3" (2011) (Comic p.19). Peter says: "We'll lickety-split it on over to his usual haunt, cross some streams, and be back in time for the news."
- ↑ Winston Zeddemore (2012). IDW Comics- "Ghostbusters Volume 1 Issue #15" (2012) (Comic p.18). Winston says: "Should we Cross the Streams?!"
- ↑ Peter Venkman (2014). IDW Comics- "Ghostbusters Volume 2 Issue #20" (2014) (Comic p.19). Peter says: "We can't always be lucky enough to save the world with five seconds of focused zapping."
- ↑ Ray Stantz (2014). IDW Comics- "Teenage Mutant Ninja Turtles/Ghostbusters Issue #3" (2014) (Comic p.3). Ray says: "Oh yeah - be careful not to cross the streams! Protonic reversal isn't on my list of things to do today!"
- ↑ Winston Zeddemore (2015). IDW Comics- "Teenage Mutant Ninja Turtles/Ghostbusters Issue #4" (2015) (Comic p.11). Winston says: "Uh, we're wedged in pretty tight, man. I can't reach my Trap without crossing your stream."