They help to create everything from spaceships to superheroes, and alien cities to entire planets.
Iron Man star Robert Downey, Jr. says visual effects artists are the "unsung heroes" behind blockbusters and Star Trek director J. J. Abrams says they do an "extraordinary amount of work".
But many visual effects (VFX) workers in the UK aren't happy.
They claim they are overworked, underappreciated and claim talented artists could leave the industry.
The union Bectu is trying to recruit British VFX workers to campaign for better working conditions.
Like many British artists, Martin Gabriel has worked on films in the Harry Potter series, which helped establish the UK as a major player in the industry.
After 18 years, he's quit: "I wasn't happy with the way people are being treated.
"However exciting the industry is, it does have this terrible dark secret."
He says that while Hollywood studios make hundreds of millions of pounds from blockbusters, VFX artists end up working unlimited unpaid overtime.
"[The studios] do not pay enough for visual effects," he said. "They seem to have an idea that it's just people pressing buttons on a keyboard, which ignores the fact that it's an incredibly skilled thing which requires a huge amount of talent.
"The way to solve this is to hire more people and have a shift system."
Several working VFX artists agree with Martin but were not willing to be interviewed.
One described the industry as a "mess", saying "staff are all too often required to work insane hours".
Another wrote: "Exploitation is an understatement to be honest."
But British firms are in competition with others around the world, so have to offer a competitive price to win bids to work on movies.
Working conditions for British VFX workers are thought to be better than those in other countries.
Employers don't have to pay their staff for overtime and say that busy periods in the build-up to a deadline are often followed by much quieter times.
It's thought the VFX industry contributes around £250 million a year to the UK economy.
Miguel Santana is 23 and in his first year working in VFX in London.
Although he admits the working hours came as a shock, he said: "It's an incredibly exciting industry to be a part of.
"We're helping to bring some of the world's biggest films to life and you couldn't do that in any other job."
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