Fans around the world have been sharing their 1D memories, stories and pictures today, marking four years since the group were formed on X Factor.
Trending topics on Twitter have included 'How I heard about 1D', 'Thank you 1D, you changed our lives' and '4 years of 1D'.
Irish heartthrob Niall got in on the action with a series of Twitter messages expressing just how overwhelmed and thankful he is about the band's incredible support.
"I can honestly say thank you all so much, every one of you, from all over the world! Your support is absolutely incredible," he wrote.
"What you have done for us baffles my mind every day! Really appreciate everything! Thank you, love you! Let's have more fun now #4YearsOf1D."
He then uploaded a throwback picture of One Direction back when they formed on X Factor, saying: "Thank you lads, we've had a great time so far. You are family to me now, not just bandmates! Love you."
Thank you lads ,we've had a great time so far, you are family to me now , not just band mates ! Love you pic.twitter.com/IdUHD3vFnf— Niall Horan (@NiallOfficial) July 23, 2014
And how young do they all look in that picture?!
One Direction - which includes Liam Payne, Zayn Malik, Harry Styles and Louis Tomlinson - were formed on X Factor's seventh series in 2010. They initially all auditioned as solo artists before being grouped together for the live shows.
Although they didn't win X Factor, they've gone on to HUGE success all across the world, winning Brit Awards, MTV Awards, earning millions, releasing three albums and generally just being insanely amazing.
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Watch 1D's first ever music video below!