One of the most difficult achievements in world football past or present is a treble.
The 1998-99 season was a magical one for Sir Alex and his club.
Manchester United were champions of the Premier League, FA Cup and UEFA Champions League.
Winning the treble was not an easy task in any of those three competitions.
In the EPL, United took back first place with three games left to go and only beat out Arsenal by one point.
In the FA Cup, the Red Devils were forced into replays of their quarterfinal against Chelsea and semifinal against Arsenal, which is remembered due to Ryan Giggs' goal in the 109th minute that may be the most important goal that the midfielder has ever scored.
The Champions League was no walk in the park, either.
United finished second in their group to Bayern Munich and after beating Inter and Juventus in the knockout stage, they faced a rematch with Bayern.
In extra time down a goal, Teddy Sheringham and Ole Gunnar Solksjaer cemented their place in United history with a goal each to win the European title in dramatic fashion.