But there are three boys you don't know that are just as important to the group's success- their songwriters.
Carl Falk, Rami Yacoub, and Savan Kotecha are the band's foremost hitmakers.
They penned What Makes You Beautiful, One Thing, and Live While We're Young! So what's their secret?
"It felt like everyone tried to do boy bands by going to the cool, hip producers who were coming up.
We wanted to make it very vanilla. You're aiming for teens and tweens with boy-band guilty pleasure music. We weren't trying to be urban or rhythmic, and they happened to share the same vision."
Not a bad idea! Vanilla is the most popular ice cream flavor in the world!
And there's one more secret- the music itself encourages YouTube covers! Savan says:
"Each of the songs have signature riffs — something so people can play YouTube versions of our songs."
We prefer our 1D songs plain, but we guess if people want to add sprinkle to their vanilla on YouTube, we can be OK with that too.
[Image via Joel Ginsburg/WENN.]