They've topped charts across the world and starred at the VMA Awards.
Now the boys from 5 Seconds of Summer have played the iTunes Festival in London.
Fans queued outside the famous Camden Roundhouse for hours to see their idols up on stage.
They're clearly popular - but it's tough to work out who the group sound like.
There's hints of boy bands like One Direction but there's some similarities to classic pop-punk groups like Blink-182 and Green Day.
And it was these pop-punk bands that influenced the four boys.
SOUNDBITE, Musician, Ashton Irwin, saying (English):
"I was a Green Day guy, they just made me pick up my instrument and that, you know, and that's how I discovered and fell in love with music, you know? They're the bands that are responsible for our band,"
SOUNDBITE, Musician, Luke Hemmings, saying (English):
"Those kinds of bands really made bands like us, I think. And I think, I don't know, we really look up to bands like
