With Sir Alex's 25th anniversary celebrations approaching, we asked fans at Old Trafford four questions:
1) What were your thoughts when the manager first arrived?
2) What's your best memory from Sir Alex's reign?
3) What does the boss mean to you?
4) How would you sum him up?
No wonder, too. It's an amazing achievement and one that may never be repeated.
Fans have also been keen to pay their respects and share their memories, as you can see in our free video, filmed on the Old Trafford forecourt this week.
We'd love you to do the same on our Talking Reds messageboard. Tell us about your best Sir Alex memory, pen your thoughts on the secret os his succcess, or share your stories, if you're lucky enough to have one, of the time you met the great man.