It's to do with a row over pay - writers say they have been losing revenue in recent years, 23% in the last two years alone.
Also, because so many series are getting shorter but writers contracts remain the same length, many of them find themselves locked into a contract without other work.
"Should this [strike] occur, writing for television, feature films and digital series will cease," the letter says.
Reality TV shows and contests like America's Got Talent would be largely unaffected
This would be catastrophic for the major networks, which were hugely affected when the WGA went on strike for 100 days in 2007.
Many TV networks struggled to fill their schedules, which led to an increase in repeats and reality TV shows.
In light of the letter, five days of negotiations are due to begin on Monday. If they don't go well - loads of your favourite shows could become victims of the strike.
Unlike the last time, many viewers will now be able to turn to streaming services to plug the gap in new content. However for traditional TV networks and film studios, a new strike would cause chaos.
Shows which are unscripted, don't use union writers or have already wrapped filming, wouldn't generally suffer. But here are some examples of shows which would.
Live entertainment and chat shows
Melissa McCarthy recently played White House secretary Sean Spicer on Saturday Night Live
The genre that would be impacted most immediately would be the live entertainment and late night chat shows.
Unlike UK talk shows fronted by the likes of Graham Norton and Jonathan Ross air weekly, most US chat shows - which are also broadcast in the UK - air every night.
As you can imagine, that requires a heck of a lot of material, and therefore a heck of a lot of writers.
The likes of Jimmy Kimmel Live, Saturday Night Live and The Tonight Show starring Jimmy Fallon would struggle to stay on air.
During the last strike, talk shows hosted by David Letterman, Jon Stewart and Conan O'Brien all ran repeats instead of airing new programmes.
Many stayed off air until the strike was over, while some returned and filled air time by ad-libbing or airing previously-recorded segments.
O'Brien, Letterman and Jay Leno even started to pay some of their regular writers out of their own pockets to provide new material during the strike.
Dramas and sitcoms
Breaking Bad and Desperate Housewives were affected by the last writers' strike
In 2007, some networks stockpiled episodes to try and minimise the impact of the writers' strike.
A few soap operas like Days of Our Lives and All My Children had their scripts completed to last them through to January, when the strike was due to end.
Dramas such as The Wire and The Shield also completed filming their current seasons and so were unaffected.
But others floundered.
Some shows were cancelled altogether, despite having ended on a cliff hanger (you can imagine how viewers reacted to that).
Others had their seasons cut short (Pushing Daisies), had new episodes delayed (24, Entourage) or their season length shortened (30 Rock, Breaking Bad, The Big Bang Theory).
Many of the shows due to begin this summer would likely be unaffected as they're already in production or completed, such as Veep, Fear The Walking Dead and Orange Is The New Black.
But shows which are due to begin in the crucial fall season, which writers begin working on in May and June, are at severe risk.
As the new letter from the WGA states: "Any delay in the start of work has the potential to postpone fall season premieres and reduce the amount of new programming available to advertisers and audiences."
This would affect brand new shows as well as programmes already hugely popular with audiences like Modern Family and Empire.
Feature films
Quantum of Solace starred Olga Kurylenko and Daniel Craig
Of course, a strike wouldn't just impact TV shows - films can suffer as well.
Perhaps the most obvious casualty of the last one was the slightly dodgy James Bond film, Quantum of Solace.
Hot on the heels of the hugely popular and critically acclaimed Casino Royale, expectations for Daniel Craig's second Bond film were high.
But the actor famously ended up writing some sections of his own dialogue because of the lack of writers available to work on the movie.
The franchise redeemed itself with Skyfall and Spectre, but other standalone films didn't have that luxury.
Johnny Depp's Shantaram never saw the light of day
Johnny Depp adventure drama Shantaram was put on hold because of strike related script issues and was never made.
Oscar-nominated musical Nine, which starred Nicole Kidman, Daniel Day-Lewis and Fergie from the Black Eyed Peas, was severely delayed.
Production had to be postponed from early 2008 to the second half of the year, pushing back its scheduled release date.
And at the height of awards season, the 2008 Golden Globes were cancelled with the winners read out at a press conference as there was no one to write the host's jokes.
Thankfully a deal was reached in time to allow stars to dress up and go to the Oscars.
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